sql, opt: support hint to disallow full scan, update coster to avoid full scans

Exalate commented:

https://github.com/cockroachdb/cockroach/pull/71317 --- Release note (sql change): Fixed an oversight in which a full scan of a partial index could be rejected due to the disallow_full_table_scans setting. Full scans of partial indexes will no longer be rejected if disallow_full_table_scans is true, since a full scan of a partial index must be a constrained scan of the table.

Jira Issue: DOC-1713