opt: add hint for skipping partial index implication

The partial index implicator makes a best-effort attempt to prove that a filter implies a partial index predicate. But it is not designed to prove implication 100% of the time—doing so would be too expensive. Therefore, it may be useful to have a hint or session setting that allows partial index implication to be skipped for a specific index such that a constrained scan is always planned on it. This setting can be dangerous—using it incorrectly would produce incorrect query results—so it should be left undocumented.

Jira issue: CRDB-37897