kvserver,sql: allow queries to ignore circuit breaker errors

[erikgrinaker]

We recently added replica circuit breakers (#71806), which will cause requests against unavailable ranges to return ErrBreakerOpen errors instead of hanging forever.

In an unavailable range scenario we often want to access whatever data we can (i.e. on all other ranges) to export it into a new cluster. It would be useful if e.g. scans, exports, and backups had an escape hatch where they would simply ignore ErrBreakerOpen and omit the unavailable data, providing an easily accessible, rudimentary best-effort recovery option.

Jira issue: CRDB-13616

Epic CRDB-39898

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