[test framework] Add a facility to format an uint256 in hex

Summary: This is a long overdue factorization of the f strings that are scattered in all the functional tests to print/call rpcs on hashes.

Test Plan:

ninja all check-functional-extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11868