Expose SHA256 low-level API

[Shigoto-dev19]

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 Add partial SHA256 hash gadget #1668.

Changes

• 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.

[mitschabaude]

for visibility - some feedback given via DM

[Trivo25]

Looks good so far, no major concerns except cleaning up and moving around the otherwise unused constants in the original method

[Trivo25]

And, feel free to take the honor of adding your changes to the changelog!

[mitschabaude]

great work!