History sharing: Handle devices we can't establish an Olm session with

If Alice invites Bob to a room, but is unable to claim OTKs or a fallback key for one or more of Bob's devices, she will be unable to share history with those devices.

Proposed solution: Send a "withheld" to-device message saying "no Olm" and another field explaining what we are trying to do. Receiver should display UTDs in this room as "There was a problem being sent the keys for this room." Store this information locally to the receiving client - in future we might consider adding this info to the backup so later devices can see why they don't have history.

Unless we download all messages just to mark them as UTD, we can only store a timestamp (even though that is bad) saying "everything before this is covered by the problem".

Part of https://github.com/element-hq/element-meta/issues/2829

No files, tests, or entry points are named. Start by tracing history sharing, Olm session or key-claim failures, and withheld to-device message handling; done means affected devices receive the stated reason, clients store a timestamp covering earlier history, and UTDs display the proposed room-specific message.

Written by the indexing model from the issue text.