feat(bb): output proof and public inputs as separate files when generating proof

[saleel]

Right now bb prove command outputs proof and publicInputs in one file which makes it difficult to parse them separately, especially when needing them for Solidity verifier.

To make things even harder, the way proof and publicInputs are encoded are different for UltraPlonk vs UltraHonk backend.

Saving them as separate files would make things easier. Also remains consistent with bb.js API.