Add prover parameters file and set defaults to 96 bits

[m-kus]

Why: currently there's no way to change PCS config externally, plus the default parameters set to 15 bits which may mislead ppl benchmarking programs with Stwo (e.g. in terms of the proof size).

What:

• ProverParameters struct is introduced (similarly to Stone which has prover config and prover params)
• A new CLI optional argument is introduced
• Default prover parameters chosen to ensure 96 bits of security and to keep the same proving time (prioritize speed over proof size)

This PR depends on #419
Related to #410

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This change is 

[m-kus]

@yuvalsw please take a look