[Chronik] Add missing ArrayToHash function

Summary:
This was accidentally removed while rebasing some commits before landing, which breaks the Chronik build.

Adding it back fixes this.

Test Plan: ninja && ninja test_bitcoin && ./src/test/test_bitcoin -t chronikbridge_tests

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13308