release-22.2: grpcutil: consider WaitingForInitError as unambiguous

[erikgrinaker]

Backport 2/2 commits from #100213.

Release justification: retry certain fatal startup errors.

/cc @cockroachdb/release

---

Previously, WaitingForInitError could be considered ambiguous to a 1PC write in DistSender. This could cause synchronous writes during node startup to fatally error out if they encountered such a node. This may have become a more common failure mode in roachtests following a788bfb, which introduced such a write early during node startup.

This patch marks such errors as unambiguous, allowing the DistSender to retry them.

Resolves #99523.
Resolves #99568.

Epic: none
Release note: None

[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