Part of #33007.

In #74500 we propose adding structure to the errors returned from unavailable Replicas on fail-fast. Fail-fast errors will be marked with a structured ReplicaUnavailableError containing (at least one) key span for which the surrounding Replica failed fast due to an open circuit breaker. These spans can be converted to SQL table descriptors and a descriptive message (and perhaps error code) should be emitted to users, and should be documented as well.

Jira issue: CRDB-12120