Implement pluggable password auth

Allows delegating the password auth to an external module. This also
moves the LDAP auth to using this system, allowing it to be removed from
the synapse tree entirely in the future.

tests/utils.py

@@ -52,6 +52,7 @@ def setup_test_homeserver(name="test", datastore=None, config=None, **kargs):
         config.server_name = name
         config.trusted_third_party_id_servers = []
         config.room_invite_state_types = []
+        config.password_providers = []
 
     config.use_frozen_dicts = True
     config.database_config = {"name": "sqlite3"}