REST: add "blockhashbyheight" call, fetch blockhash by height

Summary:
[QA] add rest tests for /rest/blockhashbyheight/<HEIGHT>.<FORMAT>
[Docs] add short documentation for /rest/blockhashbyheight

This is a backport of Core PR14353

Test Plan: ninja && ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7681