Create the pg_ident_rule defined type

This allows us to declare user map as easilly as pg_hba entries.
This file previously was unmanaged; if `$manage_pg_ident_conf` defaults to true then this could potentially wipe out previous user configuration, which sounds backwards compatible to me :(.

Could we make this default to false for now and update the readme to mention that this will default to true in the next major release?