This change adds a new `sql` operation, that allows to define an `up`
SQL statement to perform a migration on the schema.
An optional `down` field can be provided, this will be used when trying
to do a rollback after (for instance, in case of migration failure).
A new trigger is installed to capture DDL events coming from direct user
manipulations (not done by pg-roll), so they are stored as a migration,
getting to know the resulting schema in all cases.
