Create a mypy plugin for replication class methods

[anoadragon453]

Replication servlet classes, such as ReplicationRemoteJoinRestServlet, ReplicationUserDevicesResyncRestServlet etc. each have _serialize_payload and _handle_request methods. Traditionally we haven't added typing to these methods as it breaks mypy for two reasons:

• Their signatures differ from the base methods in ReplicationEndpoint.
• Their signatures differ from send_request, which is what is returned by make_client and what is run when making a replication request.

If types are added to any instance of _serialize_payload or _handle_request, mypy will complain about this discrepancy, and thus an # type: ignore has typically been put on the method signature in order to avoid this:

synapse/synapse/replication/http/membership.py

Lines 55 to 62 in 377673d

	@staticmethod
	async def _serialize_payload( # type: ignore
	requester: Requester,
	room_id: str,
	user_id: str,
	remote_room_hosts: List[str],
	content: JsonDict,
	) -> JsonDict:

A better solution is to write a mypy plugin, much like we already have for cache functions to properly check the types and remove the need to ignore them.

Note that this issue grew out of a discussion here: #8809 (comment)