release-21.2: server: add basic TLS support to tenant HTTP server

[blathers-crl]

Backport 1/1 commits from #69723 on behalf of @dhartunian.

/cc @cockroachdb/release

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Previously, the tenant HTTP server only served non-TLS connections by
default. This change will use the TLS config when it's present and serve
the HTTP server over TLS on the tenant. As on dedicated nodes, the
configuration will pick up the optional UI cert or the node cert as a
fallback.

This change also adds TLS to all other HTTP servers on the tenant server
including the _status/vars endpoint and pprof endpoints.

Resolves #69927

Release justification: fixing high-priority bug in existing
functionality.

Release note (security update): sql tenant servers will now use a TLS
certificate for their HTTP server when it's present. Previously this
server never used TLS.

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Release justification: This change is necessary to support TLS for HTTP on tenant instances. It will only be enabled for tenants running in secure mode. No changes to kv nodes are made in this PR.

[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 

[chrisseto]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @dhartunian and @knz)