fuzz: Add process_messages harness

Summary:
Backport of core PR18521.
Depends on D8374.

Test Plan:

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

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8376