Add fuzz testing for BlockTransactions and BlockTransactionsRequest
Summary:
As per title.
Backport of core PR10409:
https://github.com/bitcoin/bitcoin/pull/10409/files
Test Plan: Build and run the fuzzer.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4580