Add constraints for hashing in Honk stdlib Transcript

[ledwards2225]

For expedience, and since the hash used for fiat-shamir is likely to change, challenge generation in the Honk stdlib Transcript is "stubbed out" and does not add constraints. Resolving this issue means either adding constraints for the existing pedersen-blake3s hash if it becomes useful or adding constraints corresponding to the new hash, e.g. poseidon.