Function prove_respondent 

pub fn prove_respondent(
    q: &BigUint,
    paillier: &PaillierPublicKey,
    ck: &CommitmentKey,
    c1: &BigUint,
    c2: &BigUint,
    x: &BigUint,
    y: &BigUint,
    r: &BigUint,
) -> RespondentProof

Prove the responder statement for c₂ = c₁^x·Γ^y·r^N mod N², where r is the Paillier randomness used for the Γ^y term.