release-21.2: cli: add --max-sql-memory flag to cockroach mt start-sql

[jaylim-crl]

Backport 1/1 commits from #71011.

/cc @cockroachdb/release

---

Previously the --max-sql-memory flag wasn't available to the multi-tenancy
start-sql command, even though the feature was already there for other
start-related commands.

Release note (cli change): cockroach mt start-sql will now support the
--max-sql-memory flag to configure maximum SQL memory capacity to store
temporary data.

Release justification: The upcoming Serverless MVP release plans to use a
different value for --max-sql-memory instead of the default value of 25%
of container memory. This commit is only a flag change that will only be used
in multi-tenant scenarios, and should have no impact on dedicated customers.

[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.
• 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 

[jaylim-crl]

TFTR!