release-21.2: cli: support SQL disk spilling in tenants

[blathers-crl]

Backport 1/1 commits from #71040 on behalf of @jaylim-crl.

/cc @cockroachdb/release

---

Previously, temp storage for SQL tenants was configured to point to memory
with a limit of 100MB. As a result, all ephemeral data when processing large
queries end up going to memory, and there was no way to configure this to
point to disk. This commit changes that behavior, and the default temp storage
for SQL tenants is now the same as SQL for dedicated, i.e. disk, with a limit
of 32GB. The operator can configure this through the --max-disk-temp-storage
flag.

Release note (cli change): cockroach mt start-sql will now support the
following flags to configure ephemeral storage for SQL when processing large
queries: --store, --temp-dir, and --max-disk-temp-storage.

Release note (sql change): SQL tenants will now spill to disk by default
when processing large queries, instead of memory.

---

Release justification: The upcoming Serverless MVP release plans to allow
spilling to ephemeral disk for SQL operations (See CC-4983), but the
existing start-sql for tenants doesn't support that. This commit changes
the default spilling behavior in multi-tenant scenarios, and allows an operator
to configure ephemeral SQL storage through the CLI, 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!