release-21.2: sqlproxyccl: support secure connections to SQL backends

[blathers-crl]

Backport 1/1 commits from #69959 on behalf of @chrisseto.

/cc @cockroachdb/release

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Previously, when establishing a TLS connection to the SQL backend,
the sqlproxy failed to set .ServerName on the tls.Config. The
result was the error `tls: either ServerName or InsecureSkipVerify
must be specified in the tls.Config` whenever .SkipVerify was false.
This behavior made it impossible to establish verified secure
connections to SQL backends. This commit properly sets .ServerName
based on the outgoingAddress returned by the tenantdir service.

Release note: None

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Release justification: Having a verified TLS connection betweeen the
SQLProxy and SQL Pods in Cockroach Serverless is a requirement for the
beta release. This code change enables that secure connection without
making any changes to CockroachDB, itself.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• 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.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[andy-kimball]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @catj-cockroach, @chrisseto, and @darinpp)

[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 b94d0a1 to blathers/backport-release-21.2-70290: 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 21.2.x failed. See errors above.

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