spanconfig: handle mismatched desc types post-restore

[irfansharif]

Fixes #75831, an annoying bug in the intersection between the span
configs infrastructure + backup/restore.

It's possible to observe mismatched descriptor types for the same ID
post-RESTORE, an invariant the span configs infrastructure relies on.
This paper simply papers over this mismatch, kicking off a full
reconciliation process to recover if it occurs. Doing something "better"
is a lot more invasive, the options being:

• pausing the reconciliation job during restore (prototyped in spanconfig: handle duplicate descriptor ID post-restore #80339);
• observing a reconciler checkpoint in the restore job (work since we
  would have flushed out RESTORE's descriptor deletions and separately
  handle the RESTORE's descriptor additions -- them having different
  types would not fire the assertion);
• re-keying restored descriptors to not re-use the same IDs as existing
  schema objects.

While here, we add a bit of plumbing/testing to make the future
work/testing for #73694 (using reconciler checkpoints on retries)
easier. This PR also sets the stage for the following pattern around use
of checkpoints:

1. We'll use checkpoints and incrementally reconciler during job-internal
   retries (added in spanconfig/job: improve retry behaviour under failures #78117);
2. We'll always fully reconcile (i.e. ignore checkpoints) when the job
   itself is bounced around.

We do this because we need to fully reconcile across job restarts if the
reason for the restart is due to RESTORE-induced errors. This is a bit
unfortunate, and if we want to improve on (2), we'd have to persist job
state (think "poison pill") that ensures that we ignore the persisted
checkpoint. As of this PR, the only use of job-persisted checkpoints are
the migrations rolling out this infrastructure. That said, now we'll
have a mechanism to force a full reconciliation attempt -- we can:

   -- get $job_id
   SELECT job_id FROM [SHOW AUTOMATIC JOBS]
   WHERE job_type = 'AUTO SPAN CONFIG RECONCILIATION'

   PAUSE JOB $job_id
   RESUME JOB $job_id

Release note: None

[cockroach-teamcity]

This change is 

[ajwerner]

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @adityamaru and @arulajmani)

[irfansharif]

bors r+

[craig]

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[craig]

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[irfansharif]

bors r+

[craig]

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