feat(mixin): add manual changes for pubsub and generate mixin code

[cuiy0006]

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 6.68693% with 307 lines in your changes missing coverage. Please review.

Project coverage is 93.25%. Comparing base (a695357) to head (5a25f06).

Files with missing lines	Patch %	Lines
...min/internal/subscription_admin_connection_impl.cc	0.00%	27 Missing ⚠️
...bsub/admin/internal/topic_admin_connection_impl.cc	0.00%	27 Missing ⚠️
...d/pubsub/admin/internal/subscription_admin_stub.cc	0.00%	21 Missing ⚠️
.../admin/internal/subscription_admin_tracing_stub.cc	0.00%	21 Missing ⚠️
...le/cloud/pubsub/admin/internal/topic_admin_stub.cc	0.00%	21 Missing ⚠️
.../pubsub/admin/internal/topic_admin_tracing_stub.cc	0.00%	21 Missing ⚠️
.../internal/subscription_admin_tracing_connection.cc	0.00%	18 Missing ⚠️
...b/admin/internal/topic_admin_tracing_connection.cc	0.00%	16 Missing ⚠️
...dmin/internal/subscription_admin_auth_decorator.cc	0.00%	15 Missing ⚠️
...ubsub/admin/internal/topic_admin_auth_decorator.cc	0.00%	15 Missing ⚠️
... and 13 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14756      +/-   ##
==========================================
- Coverage   93.39%   93.25%   -0.14%     
==========================================
  Files        2316     2316              
  Lines      207867   208185     +318     
==========================================
+ Hits       194139   194152      +13     
- Misses      13728    14033     +305     

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[dbolduc]

This all looks good. What is the testing plan though?

Unit testing

I think we want golden unit tests, at least in the stub that verifies that the mixin StubInterface is used, not the service's.

It looks like there is a way to define our own *.yaml for the golden files.

google-cloud-cpp/generator/integration_tests/golden_config.textproto

Line 72 in a695357

override_service_config_yaml_name: "generator/integration_tests/test_request_id.yaml"

Then we would add a unit test here:

https://github.com/googleapis/google-cloud-cpp/tree/main/generator/integration_tests/tests

Integration Testing

I think we also need an integration test that proves that these APIs work against production. We typically use spanner's admin APIs for integration testing the generator.

It looks like we can eventually have something for the LRO mixin.

https://github.com/googleapis/googleapis/blob/78d1fb208e4af3022a0aaf27bafa578ff326326e/google/spanner/admin/database/v1/spanner.yaml#L7

---

Also, this API is now obsolete:

google-cloud-cpp/google/cloud/pubsub/options.h

Lines 415 to 429 in a695357

	/**
	* Convenience function to initialize a
	* `google::cloud::iam::IAMPolicyConnection`.
	*
	* To manage the IAM policies of Pub/Sub resources you need to configure the
	* `google::cloud::IAMPolicyClient` to use `pubsub.googleapis.com` as the
	* `google::cloud::EndpointOption` and `google::cloud::AuthorityOption`.
	*
	* This function returns an object that is initialized with these values, you
	* can provide additional configuration, or override some of the values before
	* passing the object to `google::cloud::iam::MakeIAMPolicyConnection`.
	*
	* @ingroup google-cloud-pubsub-options
	*/
	Options IAMPolicyOptions(Options opts = {});

We should deprecate it, and recommend that applications use the new mixed-in APIs. Can you open an issue?

[cuiy0006]

I created an issue for adding tests for mixin.

#14762