MSC2399: Reporting that decryption keys are withheld

[BillCarsonFr]

See matrix-org/matrix-spec-proposals#2399

When an encrypted message is sent in a room, the megolm key might not be sent to all devices present in the room. Sometimes this may be inadvertent (for example, if the sending device is not aware of some devices that have joined), but some times, this may be purposeful. For example, the sender may have blacklisted certain devices or users, or may be choosing to not send the megolm key to devices that they have not verified yet.

Currently, when this happens, there is no feedback given to the affected devices; devices that have not received keys do not know why they did not receive the key, and so cannot inform the user as to whether it is expected that the message cannot be decrypted. To address this, this proposal defines a message that senders can (optionally) send to devices indicating that they purposely did not send a megolm key.

A similar issue happens with keyshare requests; devices are not informed when other devices decide not to send back keys, and so do not know whether to expect to receive a key in response to the request.