release-21.2: [CRDB-2743] sql: create builltin generator crdb_internal.probe_range

[Santamaura]

Backport 1/1 commits from #79546.

/cc @cockroachdb/release

---

Previously there was difficulty in diagnosing kv layer health when
an incident occurs. This patch introduces the new virtual table
crdb_internal.probe_range which utilizes the kvprober to probe
each range to determine if the range can be reached or not.

resolves #61695

Release note: None

Release justification: Observability QoL improvement

[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 

[rafiss]

@Santamaura do you plan to merge this, or have you decided to hold off?

[Santamaura]

@rafiss this one is blocked from merging as there are some indirect changes to EvalContext and Span that haven't been back-ported (haven't gotten around to finding someone to back-port those changes/make the changes myself)

[rafiss]

Thanks! Given that it's been this long and 21.2 will have no more releases after November, it doesn't feel that urgent to get this in.