Allow export of subsets of room encryption keys based on a predicate

[dkasak]

The most important feature would be to export room encryption keys per room. This would allow, for instance, an old member of an E2E encrypted room to supply keys to a newer member so they can read history prior to their join.

The function export_keys from the Rust SDK could be used for the implementation.

Note that there is already a /keys export command which exports all keys. The implementation of the current /keys export command is here. As a first pass, the use case in this issue could be satisfied by implementing /keys export --room-id ROOM_ID.