appservice: Add join_room_by_id(_or_alias)

[johannescpk]

Adds an appservice-specific way of joining rooms that makes sure room state is synced once (via the room /state C2S API) before the method returns so that we have a valid state before proceeding.

This still feels a bit brittle, it might actually make more sense to have a low-level functionality that checks for proper room state, that could also cover the case of syncing state for rooms that are already joined but state store was not persisted, which the regular Client does cover through sync* methods when there was no sync before the first one. But it should be OK for a first iteration and having the tests for it makes sense anyway.

Part of #228

[johannescpk]

Putting this into draft because an idea just came to mind how this maybe could get solved directly near the the store level instead of "overwriting" individual methods like join_room_by_id. Basically, when a transaction event comes in, the state for a room is "empty" and we're in appservice mode, then make sure to call the /state API for the room once to refresh its state, which should also cover the "already joined but state store wasn't persisted" case. Have to see whether that works out.

[johannescpk]

Superseded by #303