Function add_relay_profile 

pub fn add_relay_profile(
    user_id: &str,
    device_id: &str,
    token: &str,
    relay_url: &str,
    hostname: &str,
    label: Option<&str>,
    device_private_key: &str,
) -> Result<String, CredentialError>

Add a new RelayProfile to MultiConfig for a freshly-authenticated relay. Used by the deep-link callback when PendingRelayAdd is set. Returns relay_id.