Add partial_sha256 API

[jp4g]

Problem

DKIM and JWT signatures (for example) often use signatures over SHA256 hashes to validate payload authenticity. A common pattern to save proving resources is to natively partially compute the hash state up to the point a circuit needs to access the preimage. The circuit would start from this state and complete the hash with the remaining blocks given to it. This is essentially free efficiency for circuit developers and is possible with the sha256_compression opcode.

Happy Case

ZKEmail.nr has partial hashing already integrated. We can simply move this API into the official noir-lang/sha256 package

Workaround

None

Workaround Description

No response

Additional Context

No response

Project Impact

None

Blocker Context

No response

Would you like to submit a PR for this Issue?

Yes

Support Needs

I can PR this if maintainers want to add this functionality - just confirm or deny