[Chronik] Add load_raw_tx to chronik-bridge

Summary:
Allows us to efficiently get the raw tx data without having to use load_tx and then serialize.

Depends on D13830.

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

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13831