Add `StreamKeyType` class and replace string literals with constants (#12567)

changelog.d/12567.misc

synapse/handlers/account_data.py

synapse/handlers/appservice.py

synapse/handlers/device.py

synapse/handlers/devicemessage.py

synapse/handlers/initial_sync.py

synapse/handlers/pagination.py

synapse/handlers/presence.py

synapse/handlers/receipts.py

synapse/handlers/room.py

synapse/handlers/search.py

synapse/handlers/sync.py

synapse/handlers/typing.py

synapse/notifier.py

synapse/replication/tcp/client.py

synapse/server_notices/server_notices_manager.py

synapse/storage/databases/main/e2e_room_keys.py

synapse/storage/databases/main/relations.py

synapse/types.py