Add a pre-shared key encryption option

[35609902357]

Add an option for users to have a pre-shared key (how the key will be shared is up to them) to add quantum resistant encryption. It should also be possible to add the symmetric key at a later time (e.g. two users begin to chat normally, then they meet in person and they add a symmetric key to add post-quantum resistance to all future messages/calls

[chaserene]

thanks for this issue, I've been wondering whether this is already possible in Element. this is important to build because, until post-quantum cryptography is standardized and implementations are safe and efficient, this is the only way to ensure the future privacy of e2ee communications.