Add to_*_event_stub methods to pdu, filter for correct event kind

When creating some responses (sync) an AnyRoomEventStub is needed for this
PduEvent will deserialize the JSON as a Stub event and a non stub event
when needed. Ephemeral and account events are checked to be the correct
type and filtered out if not. This requires an extra `deserialize` call
which could/should be removed.

TODO: Possibly get rid of EventJson in some places.

src/database/rooms/edus.rs

@@ -62,6 +62,8 @@ impl RoomEdus {
         room_id: &RoomId,
         since: u64,
     ) -> Result<impl Iterator<Item = Result<EventJson<EduEvent>>>> {
+        // TODO is this                                ^^^^^^^
+        // only ever a read receipt could we just return EphemeralRoomEvent here?
         let mut prefix = room_id.to_string().as_bytes().to_vec();
         prefix.push(0xff);