[Chronik] Add ChronikBridge::load_block

Summary:
Loads a CBlock from disk given a CBlockIndex. We need this to index the existing tx data from the node.

Depends on D13292.

Test Plan: 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/D13293