Create a verifier without the need of access to the registered parcipants

[iquerejeta]

Now, signatures can only be verified by an StmClerk

pub struct StmClerk<H, E>
where
    H: MTHashLeaf,
    E: ProverEnv,
{
    avk: Arc<MerkleTree<H>>,
    params: StmParameters,
    total_stake: Stake,
    proof_env: E,
    proof_key: E::ProvingKey,
    verif_key: E::VerificationKey,
}

The avk is represented as the full MerkleTree. However, for the proof verification, we only need the merkle commitment. To this end, we'll build a new entity, Verifier, which will only contain the minimal data needed to verify aggregated signatures.