release-22.1: backupccl: make checkpoint interval configurable

[stevendanna]

Backport 1/1 commits from #83151.

/cc @cockroachdb/release

---

This PR makes the interval between checkpoints configurable and also
excludes the processing time of the checkpoint itself from that
interval.

The goal of this change is to potentially address issues we've seen in
large clusters that we currently believe can be attributed to the
backup process slowing down substantially once it takes a minute or
longer to marshall, compress, and write the progress checkpoint.

Release note (ops change): A new setting
bulkio.backup.checkpoint_interval controls the minimum interval
between writes of progress checkpoints to external storage.

Release justification: Bug fix for bug that prevents backup up large
databases.

[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