[Chronik] Add get_block_header to FFI

Summary: Returns the serialized block header of a given CBlockIndex, and expose it via FFI to Chronik.

Test Plan: ninja test_bitcoin && ./src/test/test_bitcoin -t chronikbridge_tests

Reviewers: PiRK, #bitcoin_abc

Reviewed By: PiRK, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16442