Add a message ID to most of the to-device contents

[poljar]

To-device events are hard to track over federation, see element-hq/element-meta#558 for more info.

We should do something similar as matrix-org/matrix-js-sdk#2938 for the rust-sdk. The m.room_key events are the most important ones that should receive such a message ID.

[ara4n]

This is fairly urgent, presumably, given ER & EX now flying around in the field, and the fact that EI/EA/EW all have it already?