Currently, the changefeed nemeses system does not support schema
changes. This has previously allowed some fairly critical bugs to creep
past our testing.

This PR adds support for schema change events to the changefeed nemeses.

Release note: None.

Currently, the changefeed poller detects a scan boundary when it
detects that the last version of a table descriptor has a pending
mutation but the current version doesn't. In case of an `ALTER TABLE
DROP COLUMN` statement, the point at which this happens is the point at
which the schema change backfill completes. However, this is incorrect
since the dropped column gets logically dropped before that.

This PR corrects this problem by instead using the set of column
descriptors within the current and previous table descriptors to detect a
scan boundary.

Fixes cockroachdb#41961

Release note (bug fix): Changefeeds now emit backfill row updates for
                        dropped column when the table descriptor drops
                        that column.

The cloud storage sink uses the original filename format, so unlike
release-19.2, we also need to pad schemaID.

Release note: None