InvisibleCrypto | Share Room Keys: new RoomKeyShareStrategy based on user identities instead of per device

[BillCarsonFr]

Follow up of #3562
Part of matrix-org/matrix-spec-proposals#4153.

Introduce a new strategy to collect recipients for room_keys.
New strategy will consider user identities and only distribute to devices signed by their owner

Notes from https://github.com/element-hq/crypto-internal/issues/307:

Introduce a new implementation of the new RoomKeyShareStrategy trait based on Indentities instead of the existing one based on devices. + tests