[rpc] add getfinalizedblockhash to return the current finalized block

Summary:
As per title. Add an API so that node operators can introspect the currently
finalized block in case they need to make manual adjustments.

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, schancel

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D2114