Add a `generate_challenges` method on Plonky3 directly

[LindaGuiga]

Currently, we implement get_challenges, which gets all the challenge values in advance. But this is duplicating code from Plonky3's verifier, and we should therefore implement a public method on Plonky3 directly.

[Nashtare]

I think it'd make more sense to open an issue in Plonky3 repo instead