[Chronik] Add get_block_ancestor for CBlockIndex::GetAncestor

Summary:
Allows us to get an ancestor of a &CBlockIndex in Rust.

Depends on D13288.

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/D13289