[Chronik] Make ChronikBridge::get_chain_tip return CBlockIndex

Summary:
Combined with get_block_info, we can get the same data, but this way, we can get BlockInfo of any CBlockIndex.

Depends on D13290.

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/D13291