* add sha256 bench

* feat: Refactor sha256 benchmark and update dependencies

- Refactored the `sha256.rs` benchmarking file by replacing the former `hex_literal::hex` usage with a newly created `decode_hex` function.
- Updated the "Test vectors" section to work with the new `decode_hex` function to compute digests in `Sha256Circuit` instances.
- adapted to new APIs for RecursiveSNARK
- Amended the `Cargo.toml` file to include the `sha2` package dependency, version `0.10.7`.
- added the bench target

* cargo fmt

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Co-authored-by: Srinath Setty <srinath@microsoft.com>