[Chronik] Add /blockchain-info HTTP endpoint

Summary: Allows to query some info about the state of blockchain. Currently this is just the tip hash and height.

Test Plan: ninja && ./test/functional/test_runner.py chronik_blockchain_info

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13829