Function sign_envelope 

pub fn sign_envelope(
    env: &mut ChangesetEnvelope,
    keypair: &UserKeypair,
    changeset_bytes: &[u8],
)

Sign a changeset envelope with the user’s Ed25519 keypair.

Sets author_pubkey to the hex-encoded public key and signature to the hex-encoded detached signature over the envelope metadata and changeset bytes (see signing_payload).