tests: Add fuzzing harness for MessageSign, MessageVerify and other functions in util/message.h

Summary: Backport of core PR18783.

Test Plan:

ninja bitcoin-fuzzers
./src/test/fuzz/message <path_to_corpus>

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9050