ccl/sqlproxyccl: add PROXY protocol support via CLI flag to sqlproxy

[jaylim-crl]

This commits adds a new require-proxy-protocol flag to mt start-proxy, and
that changes the sqlproxy's behavior to support the PROXY protocol. When the
flag is set, the protocol will be enforced on the SQL listener, and supported
on a best-effort basis on the HTTP listener. If the PROXY protocol isn't used,
but is enforced, the connection will be rejected. The rationale behind doing
best-effort basis on the HTTP listener is that some healthcheck systems don't
support the protocol.

This work is needed for the AWS PrivateLink work in CockroachCloud, which
requires the use of the PROXY protocol.

Release note: None

Epic: none

Release justification: SQL Proxy change only. Changes are needed for the AWS
PrivateLink work in CockroachCloud.

[cockroach-teamcity]

This change is 

[jaylim-crl]

I'll figure out how to fix TestProxyProtocol/allow=false later today. Couldn't seem to reproduce it:

    proxy_handler_test.go:139:
          Error Trace:  github.com/cockroachdb/cockroach/pkg/ccl/sqlproxyccl/proxy_handler_test.go:139
                              github.com/cockroachdb/cockroach/pkg/ccl/sqlproxyccl/proxy_handler_test.go:201
          Error:        Received unexpected error:
                        Get "http://127.0.0.1:45243/_status/healthz/": readLoopPeekFailLocked: <nil>
          Test:         TestProxyProtocol/allow=false

[jaylim-crl]

This PR is ready for a review as the flakes have been fixed.

[pjtatlow]

LGTM!

[jaylim-crl]

TFTR!

bors r=pjtatlow

[craig]

This PR was included in a batch that was canceled, it will be automatically retried

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 374ed5e to blathers/backport-release-22.2-99429: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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