Add support for knocking to workers. (#15133)

changelog.d/15133.feature

docker/configure_workers_and_start.py

docs/workers.md

synapse/handlers/room_member.py

synapse/handlers/room_member_worker.py

synapse/replication/http/membership.py

synapse/rest/__init__.py

synapse/rest/client/knock.py

synapse/rest/client/room.py