release-23.2: sqlproxyccl: add proxy protocol listener

[DuskEagle]

Backport 1/1 commits from #117241.

/cc @cockroachdb/release

Release justification: high-priority business need for the functionality; the new functionality is only accessed by a startup flag; the new functionality only affects SQLProxy, not CRDB itself.

---

To support GCP Private Service Connect, we need to have a listener in
SQLProxy which expects packets to contain proxy protocol headers. This
listener will be used for all traffic inbound from PSC. At the same
time, SQLProxy must continue to accept connections through the public
Internet which will not contain proxy protocol headers, and for which
any proxy protocol headers we receive cannot be trusted.

This commit introduces an optional second listener in SQLProxy,
controlled by --proxy-protocol-listen-addr, which requires
proxy protocol even as the primary listener doesn't. Private Service
Connect will direct traffic to this second listener.

Resolves #117240

Release note: None

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[rafiss]

i'm not familiar with the code being changes, but confirming that the backport is low risk to the main CRDB process

[jeffswenson]

LGTM

The code changed impacts the sqlproxy, which is only used in serverless deployments.