refactor: allow constructing a valid CKey in place

Summary:
allow constructing a valid CKey as syntactic sugar, less boilerplate on new code

did not change old code for now to avoid merge conflict nonsense for future backports

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9882