auth: oauth2 - cleanup db_oauth2_lookup_callback_t db parameter isn't needed, because request->db already contains it. Also moved request parameter to be the first.

auth: oauth2 - remove db_oauth2_request.failed It is an unnecessary duplication of passdb_result.

auth: oauth2 - remove db_oauth2_request.result It's not a persistent state. When it's set, the callback needs to be called. This way it's more difficult to forget to set it.

auth: oauth2 - make db_oauth2_template_export() static It's not used outside db-oauth2.c

auth: Add oauth2 passdb