Expose SHA256 low-level API

Description

This PR exposes the low-level API of the SHA-256 hash function in o1js.
It supports the development of partialSHA and dynamicSHA as external packages while leveraging the efficiencies of the low-level API.
Closes #1668.

Separated the low-level SHA-256 functions and exported them as part of the SHA-256 gadgets in o1js.
Added 2 importable functions and 1 constant:
- SHA-256 Compression Function
- Enables flexible block hashing with better control of initial and intermediate hashes.
- Message Schedule Computation Function
- Computes the message schedule from message blocks to prepare it as input to the compression function.
- SHA256 Initial State Constant
- The initial hash values, also known as the SHA256 nothing-up-my-sleeve constants, are now exposed for enhanced flexibility in constructing SHA256 hashes.