[Chronik] Send TX_FINALIZED msg for txs that are in a block that have been finalized by Avalanche

Summary:
These are sent out for all txs in a block after Avalanche finalized a block.

Unlike handle_block_connected and handle_block_disconnected, we don't get the txs in handle_block_finalized, therefore we have to load them from disk.

Test Plan: ninja && ./test/functional/test_runner.py chronik_ws_script.py

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13793