add a finalized_blockhash field to getblockchaininfo

Summary: This will be useful to automate checkpoints and UTXO snapshots based on the current finalized tip. Using the non-finalized tip for this purpose is risky as it could be reorged or parked.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18556