[chronik] add a block-header endpoint

Summary:
Fetch the header directly from the in-memory block index.

For now only the raw header data is returned. In a future diff I plan to add a cp_height query parameter that causes the endpoint to also return the checkpoint's merkle tree data (similar to how electrumx does it).

Test Plan:
With chronik
ninja all check-all

Reviewers: #bitcoin_abc, Fabien, tobias_ruck

Reviewed By: #bitcoin_abc, Fabien, tobias_ruck

Subscribers: tobias_ruck, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16508