sql: allow dropping columns referenced in partial index predicates

In #97372, a limitation was introduced that prevents a column from being dropped if it is referenced by a partial index predicate. To drop such a column, the partial indexes need to be dropped first. Ideally, we could lift this restriction.

I explored lifting this restriction in #97368, though I didn't iron out all the details. It'll be tricky to get all the edge-cases ironed out.

Jira issue: CRDB-24899

Epic CRDB-60817

Read the context from #97372 and the exploratory work in #97368 first; the payload names no implementation files or tests. Define done as allowing a column referenced by a partial-index predicate to be dropped without first dropping those indexes, while covering the unresolved edge cases described in the earlier exploration.

Written by the indexing model from the issue text.