Function sign_from_packages 

pub fn sign_from_packages<M, R: RngCore + CryptoRng>(
    message: &[u8],
    shares: BTreeMap<Identifier<BluePallas<M>>, SecretShare<BluePallas<M>>>,
    pubkey_package: PublicKeyPackage<BluePallas<M>>,
    rng: R,
) -> Result<(Signature<BluePallas<M>>, VerifyingKey<BluePallas<M>>), Error<BluePallas<M>>>
where
    M: ChallengeMessage,

Helper function to sign a message using existing key packages