Ability to select exponent in MIMC hash function

[shreyas-londhe]

Referencing to - https://github.com/ConsenSys/gnark/blob/master/std/hash/mimc/encrypt.go

I think it would be good to have the ability to select the exponent which you want for the MIMC hash inside the circuit. Currently, the library uses exponent 5 for MIMC, but many libraries for Ethereum use exponent 7.

I would love to take this up with a little bit of guidance on how I shall implement this.

I made a similar issue in gnark-crypto - https://github.com/ConsenSys/gnark-crypto/issues/221