perf: allow providing coefficients for folding KZG proofs

[ivokub]

When folding KZG proofs for verification, we hash the inputs to compute the random coefficient for computing the linear combination. However, in current use cases we already have computed the hash of inputs previously (in PLONK verifier. By allowing to provide the random coefficient from the caller (e.g. by using an option), then we can save some computation.

E.g. https://github.com/Consensys/gnark/blob/master/std/commitments/kzg/verifier.go#L486-L502