Add ServerRoomImpl to allow tests to reuse helpers with custom code

[kegsay]

With the addition of #765 it's now possible for tests to add a range of good/bad/broken rooms to Complement's internal federation server. However, these rooms cannot reuse any of the existing helper functions. To aid this, logic which was once part of the federation server itself (e.g mapping /send_join responses to internal ServerRoom state) is now transferred to a new ServerRoomImpl interface. The federation server's helper functions call this interface, allowing tests to swap out functions for their broken rooms.

For example, we may want to reuse the very convenient HandleMakeSendJoinRequests but modify the join proto event to include malformed fields. Before this PR this simply wasn't possible as we hard-coded the happy case. Tests can now do:

room := federation.NewServerRoom(gomatrixserverlib.RoomVersionV1, "!foo:localhost")
room.ServerRoomImpl = &federation.ServerRoomImplCustom{
	ServerRoomImplDefault: federation.ServerRoomImplDefault{Room: room},
	ProtoEventCreatorFn: func(def federation.ServerRoomImpl, ev federation.Event) (*gomatrixserverlib.ProtoEvent, error) {
		if ev.Type == spec.MRoomMember {
			ev.Unsigned = map[string]any{"something": "custom"}
		}
		return def.ProtoEventCreator(ev)
	},
}

[devonh]

New API functions should ideally have doc comments to explain their usage.
Even though they are pretty clear just from name & impl alone.

Otherwise these changes look promising and will make it much easier to mess around with future event changes without having to break into complement's internals.

[devonh]

Why? It seems okay to me.

[kegsay]

It's MakeRespMakeKnock so surely should be knock not join.

[kegsay]

Will chuck some comments in then merge, thanks!