release-22.2: sql: add a virtual index on database name for SHOW GRANTS ON DATABASE

[blathers-crl]

Backport 1/1 commits from #90597 on behalf of @ajwerner.

/cc @cockroachdb/release

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Before this change, we'd always fetch all databases. This is expensive if there are many. It's also the case that the databases page will run this for each database. In that case, this was an N^2 proposition.

Epic: CRDB-20865

Release note (performance improvement): SHOW GRANTS ON DATABASE and the Databases page in the cluster UI are now faster if there are many databases.

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Release justification: Simple improvement for UI behavior

[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 

[rafiss]

is this related to #90196 ?

[ajwerner]

is this related to #90196 ?

Yes. We collect the grants of the database here. In my local cluster I had 10k databases. The request would always time out because SHOW GRANTS ON DATABASE was taking 35s. I also filed #90596. After this patch, SHOW GRANTS ON DATABASE is plenty fast. SHOW GRANTS ON TABLE is still horrific. It's harder to fix but I'd like to do something.