Remove configauth default, use nil in configgrpc default (#12271)

#### Context

The `configauth.NewDefaultAuthentication` returns a zero-initialized
`Authentication` value
([link](https://github.com/open-telemetry/opentelemetry-collector/blob/477e4d3959932234e71f4e68c9d4e2290015a1df/config/configauth/configauth.go#L25)),
which is used in `configgrpc.NewDefaultClientConfig`
([link](https://github.com/open-telemetry/opentelemetry-collector/blob/477e4d3959932234e71f4e68c9d4e2290015a1df/config/configgrpc/configgrpc.go#L109))
and `configgrpc.NewDefaultServerConfig`
([link](https://github.com/open-telemetry/opentelemetry-collector/blob/477e4d3959932234e71f4e68c9d4e2290015a1df/config/configgrpc/configgrpc.go#L196)).
However, this default value is problematic, as it will cause the
Collector to crash on startup with the error `Error: cannot start
pipelines: failed to resolve authenticator "": authenticator not found`.

There is no way for the `Authentication` struct to represent "no
authentication" (which is presumably the intended default). The
`configgrpc` code uses a `nil` `*Authentication` pointer to represent
that case
([link](https://github.com/open-telemetry/opentelemetry-collector/blob/477e4d3959932234e71f4e68c9d4e2290015a1df/config/configgrpc/configgrpc.go#L310)).

Regarding the use of these APIs:
- Across Github, it looks like `configauth.NewDefaultAuthentication` is
not used outside of `configgrpc`.
- I haven't found any use of `configgrpc.NewDefaultServerConfig` on
Github, and `configgrpc.NewDefaultClientConfig` has [one use in the
Elastic OTel components
repo](https://github.com/elastic/opentelemetry-collector-components/blob/3cfc4ac3a58e5ac9823bf2ccc966e2f306cff5c9/processor/ratelimitprocessor/config.go#L164),
although I suspect the `Auth` field may get overriden anyway, so the
crash may never actually occur.
- It looks like the gRPC receivers/exporters in Core build their default
`ClientConfig`/`ServerConfig` manually instead of using those functions,
leaving the `Auth` field `nil`, which is why the crash does not occur
there either.

#### Description

This PR:
- Removes `configauth.NewDefaultAuthentication` since there doesn't seem
to be a useful "default" value for this struct. Because it doesn't seem
to be used outside this repo, I decided to skip the deprecation step.
- Replaces its use in
`configgrpc.NewDefaultClient/NewDefaultServerConfig` by `nil`. I think
this would be considered a bug fix rather than a breaking change, since
the current value causes an error if not overriden.

#### Link to tracking issue
Resolves #12223 (the original issue was about making the function
signature more consistent, but removing the function entirely makes it
no longer an issue I would say)

#### Testing
I experimentally confirmed that the above crash occurs with default
config if we change the OTLP receiver's `createDefaultConfig` to use
`configgrpc.NewDefaultServerConfig`, and that the above fix to said
function prevents the crash.

.chloggen/configauth-remove-default.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: breaking
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: configauth
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Remove NewDefaultAuthentication
+
+# One or more tracking issues or pull requests related to the change
+issues: [12223]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  The value returned by this function will always cause an error on startup.
+  In `configgrpc.Client/ServerConfig.Auth`, `nil` should be used instead to disable authentication.
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [api]

.chloggen/configgrpc-noauth-default.yaml

@@ -0,0 +1,26 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: configgrpc
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Set Auth to nil in NewDefaultClientConfig/NewDefaultServerConfig"
+
+# One or more tracking issues or pull requests related to the change
+issues: [12223]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  The value that was used previously would always cause an error on startup.
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [api]

config/configauth/configauth.go

@@ -27,11 +27,6 @@ type Authentication struct {
 	AuthenticatorID component.ID `mapstructure:"authenticator"`
 }
 
-// NewDefaultAuthentication returns a default authentication configuration.
-func NewDefaultAuthentication() *Authentication {
-	return &Authentication{}
-}
-
 // GetServerAuthenticator attempts to select the appropriate auth.Server from the list of extensions,
 // based on the requested extension name. If an authenticator is not found, an error is returned.
 func (a Authentication) GetServerAuthenticator(_ context.Context, extensions map[component.ID]component.Component) (auth.Server, error) {

config/configauth/configauth_test.go

@@ -17,12 +17,6 @@ import (
 
 var mockID = component.MustNewID("mock")
 
-func TestNewDefaultAuthentication(t *testing.T) {
-	auth := NewDefaultAuthentication()
-	assert.NotNil(t, auth)
-	assert.Empty(t, auth)
-}
-
 func TestGetServer(t *testing.T) {
 	testCases := []struct {
 		name          string

config/configgrpc/configgrpc.go

@@ -110,7 +110,6 @@ func NewDefaultClientConfig() *ClientConfig {
 	return &ClientConfig{
 		TLSSetting:   configtls.NewDefaultClientConfig(),
 		Keepalive:    NewDefaultKeepaliveClientConfig(),
-		Auth:         configauth.NewDefaultAuthentication(),
 		BalancerName: BalancerName(),
 	}
 }
@@ -196,7 +195,6 @@ type ServerConfig struct {
 func NewDefaultServerConfig() *ServerConfig {
 	return &ServerConfig{
 		Keepalive: NewDefaultKeepaliveServerConfig(),
-		Auth:      configauth.NewDefaultAuthentication(),
 	}
 }
 

config/configgrpc/configgrpc_test.go

@@ -47,7 +47,6 @@ func TestNewDefaultClientConfig(t *testing.T) {
 	expected := &ClientConfig{
 		TLSSetting:   configtls.NewDefaultClientConfig(),
 		Keepalive:    NewDefaultKeepaliveClientConfig(),
-		Auth:         configauth.NewDefaultAuthentication(),
 		BalancerName: BalancerName(),
 	}
 
@@ -83,7 +82,6 @@ func TestNewDefaultKeepaliveServerConfig(t *testing.T) {
 func TestNewDefaultServerConfig(t *testing.T) {
 	expected := &ServerConfig{
 		Keepalive: NewDefaultKeepaliveServerConfig(),
-		Auth:      configauth.NewDefaultAuthentication(),
 	}
 
 	result := NewDefaultServerConfig()