Support deterministic sigs as per RFC 6979

[libitx]

Currently K is random. Ideally default should be deterministic, perhaps with option for random if needed for some reason.

[brentongunning]

Yep we should look into this. Thanks for opening an issue. Only reason it's not in today is we'd need to write some code for HMAC and I wasn't sure how complex that would get. But ecdsaSignWithK is there for this purpose in future.