Sliding Sync: ensure `$LAZY` membership changes from state resets get sent down

[MadLittleMods]

Split out from #17929 (comment), as explained by @erikjohnston,

In state reset scenarios where a membership gets reverted to a previous state, we won't have an event in the timeline, but will have a state change in the state deltas. Currently, the $LAZY membership logic only looks in the timeline so we will miss this kind of change.

synapse/synapse/handlers/sliding_sync/__init__.py

Lines 1044 to 1063 in 8306cee

	# We need to fetch all relevant people when we're lazy-loading membership
	elif (
	state_type == EventTypes.Member
	and state_key == StateValues.LAZY
	):
	lazy_load_room_members = True
	# Everyone in the timeline is relevant
	timeline_membership: Set[str] = set()
	if timeline_events is not None:
	for timeline_event in timeline_events:
	# Anyone who sent a message is relevant
	timeline_membership.add(timeline_event.sender)
	# We also care about invite, ban, kick, targets,
	# etc.
	if timeline_event.type == EventTypes.Member:
	timeline_membership.add(
	timeline_event.state_key
	)

[MadLittleMods]

At-least, partially addressed in this PR, see #19206 (comment)