Integrate the crypto RustSDK

This issue is created to follow up the integration of the RustSDK into the matrix SDK2.

### Features / Use cases

- [x] Encrypt a message
- [x] Decrypt an encrypted message
- [x] KeysBackup support (missing in the RustSDK)
- [x] Cross-signing support
- [x] Verification support
- [x] Import and export room keys manually
- [ ] dehydrated device support
- [ ] olm fallback key support matrix-org/matrix-rust-sdk/issues/400
~~SharedSecretStorageService~~

### Other

- [ ] Import the current crypto database into the Rust crypto database, so that an upgrade of Element Android including the new RustSDK is transparent to the users
- [ ] Extract the RustSDK to a dedicated GitHub repository and add a regular dependency on it with a regular gradle dependency
- [ ] Cleanup the code on the SDK2: remove the legacy crypto code and tests