widget: Check room IDs on read-event request

Not needed by EC, but apparently it does send this field (for no good reason?). Roughly specced [here](https://github.com/matrix-org/matrix-spec-proposals/blob/travis/msc/widgets-send-receive-events/proposals/2762-widget-event-receiving.md#proposal-accessing-other-rooms). For now we can ignore it. It would make sense to

- make sure its absence is allowed in deserialization, not currently the case on the poc branch, and - raise an error if it's present but contains something other than just the current room ID (the only thing we should support until this is better specified and has a use case)

Start with the widget read-event request deserialization and compare its behavior with the linked MSC 2762 proposal. Done means an absent room ID is accepted, the current room ID is accepted when present, and other room IDs are rejected with an error.

Written by the indexing model from the issue text.