release-22.1: ccl/sqlproxyccl: rename tenant.Resolver to tenant.DirectoryCache

[blathers-crl]

Backport 2/2 commits from #78276 on behalf of @jaylim-crl.

/cc @cockroachdb/release

---

Informs #76000.

ccl/sqlproxyccl: move the TenantResolver interface to the tenant package

Previously, the TenantResolver interface lived in the base sqlproxyccl package,
and there was no way to enforce that the tenant.Directory struct implements
this interface since that would result in a cyclic dependency. This commit
moves the TenantResolver interface into the tenant package, and enforced that
the tenant.Directory struct implements that.

This would then allow us to rename the tenant.Directory struct to
tenant.directoryCache, and the tenant.Resolver interface to
tenant.DirectoryCache.

Release note: None

ccl/sqlproxyccl: rename tenant.Resolver to tenant.DirectoryCache

This commit renames the tenant.Resolver interface to tenant.DirectoryCache.
The existing Directory struct was also renamed to directoryCache to prevent
confusions since there were previously two definitions of a "directory": one
pointing to the cache of entries, whereas another referring to the actual
directory server.

Release note: None

---

Release justification: sqlproxy only change. All the changes are mechanical
changes, and are needed for the upcoming proxy load rebalancing work.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• 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 

[jeffswenson]

LGTM

[jaylim-crl]

TFTR!