Function schnorr_sign 

pub fn schnorr_sign(
    private_key: &GrumpkinScalar,
    message: &Fr,
) -> SchnorrSignature

Sign a message with a Grumpkin private key using Schnorr.

The signing algorithm matches the Noir schnorr library:

1. k = Blake2s(private_key || message) (deterministic nonce)
2. R = k * G
3. public_key = private_key * G
4. e = Blake2s(pedersen_hash([R.x, public_key.x, public_key.y]) || message)
5. s = k - private_key * e (mod Grumpkin scalar order)

Returns a SchnorrSignature containing (s, e).