feat: Implement key backups

[arcuru]

The current matrix setup reuses local keys, but doesn't backup/restore session keys. This would be useful in failure scenarios, since we'd be able to backup old session keys from the homeserver.

There's an example in the matrix-rust-sdk repo: https://github.com/matrix-org/matrix-rust-sdk/tree/main/examples/backups/src