Move Spam Checker callbacks to a dedicated file (#15453)

changelog.d/15453.misc

synapse/app/_base.py

synapse/federation/federation_base.py

synapse/federation/federation_server.py

synapse/handlers/directory.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/register.py

synapse/handlers/room.py

synapse/handlers/room_member.py

synapse/handlers/user_directory.py

synapse/media/media_storage.py

synapse/module_api/__init__.py

synapse/module_api/callbacks/__init__.py

synapse/module_api/callbacks/spamchecker_callbacks.py

synapse/server.py

tests/handlers/test_user_directory.py

tests/media/test_media_storage.py

tests/rest/client/test_rooms.py

tests/server.py