release-23.1: backupccl: skip backing up excluded spans

[dt]

Backport 1/1 commits from #108627.

/cc @cockroachdb/release

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Previously we sent export requests to all spans being backed up. The ranges for spans of tables that had set the flag to exclude data from backup would reply with an empty response, excluding their data, but the backup process still sent these ranges these requests.

This changes the backup process to not send requests for spans from excluded tables, when performing database, table, or cluster backups. Backups of tenants will still send export requests to every range for the tenant span, and those ranges that host tables that are excluded will continue to reply with no data.

This is done both as an optimization, and so that backups can succeed even when a table is unavailable, if, and only if, that table is excluded.

Release note (ops change): BACKUP now skips contacting the ranges for tables on which exclude_data_from_backup is set, and can thus succeed even if an excluded table is unavailable.
Epic: none.

Release justification: small performance improvement as well as potential improvement of reliability in partially unavailable clusters.

[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 

[dt]

With 23.1.9 already branched off release-23.1, this can backport to release-23.1 now and still will get several more weeks of bake time before being released.