[Chronik] Move proto logic from QueryTxs::tx_by_id to make_tx_proto

Summary: Having this be its own function makes the code cleaner and allows us to re-use the function at other places.

Test Plan: ninja check-crates && ninja && ./test/functional/test_runner.py chronik_tx

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13545