[chronik] add a block-headers endpoint

Summary:
This adds another endpoint to fetch a range of block headers.

The behavior is the same as the "blocks" endpoint wrt boundary checks:

• when the end height is more than the number of blocks in the active chain return less blocks than requested
• require 0 <= start_height <= end_height
• start_height and end_height are inclusive

Depends on D16508

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16526