zksch: Better API to handle NIZK and ZK

[adr1anh]

In the CMP keygen we need to perform the Schnorr proof of knowledge interactively, but for FROST this can be done in a non-interactively. This resulted in a weak API where parameters were confusing.

Instead, we now define Randomness, Commitment, Response and Proof types.

• Randomness represents the secret randomness used to generate the Commitment sent in the first round of the Sigma protocol. A prove calls r := NewRandomness() and obtains the Commitment by calling commitment := r.Commitment().
• Later, when the appropriate context was provided to the hash function, the Response is generated by calling response := r.Prove(h, public, secret). The response can be verified by calling response.Verify(h, public, commitment)
• A non-interactive Proof is a wrapper that contains a Commitment and the corresponding Response and can be validated using the same API as other ZK proofs

[adr1anh]

I'm still torn on whether Randomness.Prove(...) is the right name for the API or if Randomness.Response(...) is better.