Adding support for MSC3202 for E2E appservice support

[danpe]

Looking at matrix-org/matrix-spec-proposals#3202 seems like a pretty straight forward implementation.
I think I can contribute that part, but would love for direction on which files it should be implemented on.

[S7evinK]

Ignoring MSC2409, we need device_lists, device_one_time_keys_count and device_unused_fallback_key_types (the latter isn't implemented yet, iirc)
device_lists and device_one_time_keys_count currently have some internal functions in the SyncAPI:

dendrite/syncapi/internal/keychange.go

Line 36 in d65449c

func DeviceOTKCounts(ctx context.Context, keyAPI api.SyncKeyAPI, userID, deviceID string, res *types.Response) error {

dendrite/syncapi/internal/keychange.go

Lines 52 to 55 in d65449c

	func DeviceListCatchup(
	ctx context.Context, db storage.SharedUsers, userAPI api.SyncKeyAPI, rsAPI roomserverAPI.SyncRoomserverAPI,
	userID string, res *types.Response, from, to types.StreamPosition,
	) (newPos types.StreamPosition, hasNew bool, err error) {

Those are used for /sync responses and most likely will need to be "public" so the appservice API can use them.
So IMO, we need to modify https://github.com/matrix-org/dendrite/blob/main/appservice/consumers/roomserver.go to:

• Add OTKs for all users the appservice is interested in when the consumer receives a m.room.encrypted event
• Have appserviceState keep track of the StreamPosition (from, in this case) to be able to calculate device_lists
• Make the mentioned internal functions public