Add knock-related data to the admin export-data command

[anoadragon453]

Synapse supports exporting data about a user for administration purposes. Part of that exported data are the memberships of a user in various rooms.

Currently, this only covers join, leave, ban and invite memberships. knock (MSC2403) membership data is not currently supported:

synapse/synapse/handlers/admin.py

Lines 85 to 94 in eb9ddc8

	# Get all rooms the user is in or has been in
	rooms = await self.store.get_rooms_for_local_user_where_membership_is(
	user_id,
	membership_list=(
	Membership.JOIN,
	Membership.LEAVE,
	Membership.BAN,
	Membership.INVITE,
	),
	)

Note that this was one of the TODO's on the original knock implementation PR.

[callahad]

Leaving this open as a good issue for a first time contributor to Synapse.

Setting to P4 to suggest that paid contributors should not work on it, but patches will be accepted.