CKey::Sign : switch test arg to explicit additional entropy argument #19269

Previously a uint32_t was being given as an additional entropy to the nonce generating function whenever it was non-zero for testing purposes.

This PR allows a full 32-byte chunk of entropy to be given to the generating function. Nonce grinding continues as before by incrementing the value by one each time and re-signing.

Speculative future possible uses:

1. Mitigation to fault attacks against deterministic nonce ECDSA by grabbing fresh entropy for each signature: https://eprint.iacr.org/2017/1014.pdf
2. Sign-to-contract(swap out nonce function as well of course)

I found updating the json tests a bit annoying, so I also made a commit that I can break out into its own PR.

guess there's no interest for extremely theoretical issues :)