release-22.1: server: introduce COCKROACH_FALLBACK_SPANCONFIG_NUM_REPLICAS_OVERRIDE

[irfansharif]

It controls what replication factor is used for ranges with no explicit span configs set. This is a backportable form of the spanconfig.store.fallback_config_override we added in #92466.

Release note: None
Release justification: Introduce an env var that would've come in handy in an internal escalation -- it allows users to control what replication factor is used for ranges not explicitly referenced by zone configs. Without this env var, the value used is a hardcoded default of 3.

[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 

[tbg]

Could you add a release note telling users about the env var and intended usage? Or are we intentionally adding it "secretly"?

cc @ricardocrdb

[irfansharif]

Intentionally adding it without a release note.