schemachanger: support DROP COLUMN for PK columns

Presently, the declarative schema changer only supports dropping columns which are not part of the primary key. In postgres, dropping a PK column cascades into dropping the PK constraint also. In CRDB, the PK is tied to the primary index, therefore perhaps what we should do is rebuild the primary index using the `rowid` column as a PK.

Jira issue: CRDB-29946