Add missing escalus_user_db behaviour

Moreover:
- fix typespec broken when merging `#client{}` and `#transport{}` records
- reexport `escalus_config:config()` as `escalus:config()` (the latter is **already used** in multiple places **but not defined**)
- note that `escalus_ejabberd` ought to be a valid `escalus_user_db` implementation, but is not yet due to interface incompatibilities (please note this also touches on the return values from `escalus_users:delete/create_users` - these might need adjustment, too, but I leave this for the future).