sql/schemachanger: support complex ALTER COLUMN ... SET TYPE in declarative schema changer

[spilchen]

This is a continuation of #126143. It will continue to push down operations for ALTER COLUMN ... TYPE into the declarative schema changer. This one will deal with complex type changes. A complex type change is one where a backfill of a shadow column is needed. One example is to alter the type from a INT to a TEXT data type.

We should make sure to avoid as many of the known limitations tracked in CRDB-25314 as we can.

Epic: CRDB-25314

Jira issue: CRDB-39782

Jira issue: CRDB-40232

Epic CRDB-25314