[Chronik] Add ChronikBridge::lookup_block_index

Summary:
Allows us to get a &CBlockIndex from Rust.

Depends on D13276.

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/D13288