release-21.1: sql: add sql.mutations.mutation_batch_byte_size setting

[yuzefovich]

Backport 1/1 commits from #67537.

/cc @cockroachdb/release

Release justification: allows for restoring backups with wide rows.

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Previously we always constructed 10k row insert batches, regardless of the
size of those rows. With large rows, this could easily exceed the kv size
limit of 64MB. This changes batch construction to track the size of added
keys and values, and send the batch either when it has 10k entries of when
the size of added keys and values exceeds the setting, which defaults to 4MB.

Fixes #67542.

Release note (bug fix): INSERT and UPDATE statements which operate on larger rows are split into batches using the sql.mutations.mutation_batch_byte_size setting.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• 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.

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 

[yuzefovich]

Note that I squashed #67930 into the commit.