migration_listener: Add empty base class and "before" callbacks for tables
migration_listener: Add empty base class and "before" callbacks for tables

Empty base type makes for less boiler plate in implementations.
The "before" callbacks are for listeners who need to potentially
react/augment type creation/alteration _before_ actually
committing type to schema tables (and holding the semaphore for this).

I.e. it is for cdc to add/modify log/desc tables "atomically" with base.

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla