spanconfig: add helpful crdb_internal builtins

[irfansharif]

Is your feature request related to a problem? Please describe.

The following builtins may prove to be useful to have around if diagnosing issues with span configs infrastructure. The ability to:

• recompute tenant-side limit (system.span_count)
• force restart the reconciliation job
• disable the reconciliation job
• force restart the kvsubscriber process
• introspect the kvsubscriber cache

Jira issue: CRDB-15833

[irfansharif]

To force restart the reconciliation job and disable the reconciliation job, you can just use the jobs infra.

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

PAUSE JOB $job_id
RESUME JOB $job_id

[irfansharif]

Tracking in #81009, gonna be best-effort/friday work from now.

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!