[Chronik] Add get_block_info

Summary:
Return some relevant info from a CBlockIndex (hash, height).

Depends on D13291.

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/D13292