sql: avoid locked read during FK check of global table

Under RC isolation, FK checks use locked reads in order to avoid anomalies that would invalidate the FK check. But, these locked reads require a trip to the leaseholder replica.

If the FK we're checking is in a global table, we might not need to go all the way to the leaseholder. It might suffice to check the region-local replica and rely upon the properties of the global table to avoid anomalies.

Jira issue: CRDB-57680