Add SHA-256 starky

[tumberger]

Currently, SHA-256 in starky is not included in the benchmarks.

There is a fork of plonky2 that includes sha256 in the starky src directory (based on this previous fork that has been used in the benchmarks as published by celer network). However, there is no concrete test for correctness in this implementation.

Adding SHA-256 with a test for correctness for starky in the current set of benchmarks would be a great addition.