sql: add a way to control index choice in post-queries

[yuzefovich]

We currently support index hints in the main query (as well as subqueries (?)), and it would be nice if we had a way to control the plan in the post-queries. For example, we might want to force usage of the PK in the FK check query.

The main difficulty will be coming up with the UX around this. One idea could be utilizing the external query plan hints that Drew has the prototype for; another way could be annotating the FK constraint itself in the schema somehow. This would have been useful as a workaround during this support ticket.

Jira issue: CRDB-47946