feat(ivc): ivc circuit with private inputs

[chaosma]

In current Sirius, the step circuit F doesn't have private input. i.e. it assumes that z_{i+1}=F(z_i). However, in practice, e.g. in zkEVM circuit, we will encounter private input as: z_{i+1}=F(z_i, w_i).

This issue is somehow related to #316 where both issues need to modify the StepInputs and StepFoldingCircuit etc. We might do them together.

[updated]: Actually, the private input is passed along with sc1 in fn fold_step(pp, sc1, sc2) and used during sfc.synthesize. I overlooked step_circuit field in StepFoldingCircuit.