Join a group from a Welcome message

To join a group from a Welcome message, a new MlsGroup can be instantiated directly from the Welcome message. If the group configuration does not use the ratchet tree extension, the ratchet tree needs to be provided as well.

    let mut bob_group = MlsGroup::new_from_welcome(
        backend,
        &mls_group_config,
        welcome,
        None, // We use the ratchet tree extension, so we don't provide a ratchet tree here
    )
    .expect("Error joining group from Welcome");