release-22.2: backupccl: elide expensive ShowCreate call in SHOW BACKUP

[adityamaru]

Backport 1/1 commits from #88293.

/cc @cockroachdb/release

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In #88376 we see the call to ShowCreate taking ~all the time on a cluster with
2.5K empty tables. In all cases except SHOW BACKUP SCHEMAS we do not
need to construct the SQL representation of the table's schema. This
results in a marked improvement in the performance of SHOW BACKUP
as can be seen in #88376 (comment).

Fixes: #88376

Release note (performance improvement): SHOW BACKUP on a backup containing
several table descriptors is now more performant

Release justification: low risk performance improvement required for the use of schedules in CockroachCloud

[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 

[adityamaru]

blathers backport 22.1

[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 fdf45ed to blathers/backport-release-22.1-88711: 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 22.1 failed. See errors above.

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