feat: allow computing Groth16/PLONK proofs from the solution directly without invoking the solver

Currently we can compute the proof only from the initial witness from which gnark solver computes the full solution. But for fuzzing/testing purposes it would be good if we could compute the proof directly from the solution, even if the proof would be incorrect.

See also https://github.com/Consensys/gnark/pull/1474 and https://github.com/Consensys/gnark/discussions/1513#discussioncomment-13561643