Add support for dehydrated devices

MSC: https://github.com/uhoreg/matrix-doc/blob/shrivelled_sessions/proposals/3814-dehydrated-devices-with-ssss.md

This will help resolve some edge-cases where UTD errors can arise in the following cases:

1. All devices for a user have been logged out
2. On room invite, MSC3061 key forwarding forwards old room keys to a user A's device 1. User A's device goes offline and User A creates device 2. Device 2 will not get the forwarded keys