[Chronik] Move load_(raw_)tx into ChronikBridge

Summary:
Similar reasoning to D15537, this parallels the other load_ functions but for txs and doesn't block backporting BlockManager from Core on Chronik.

Depends on D15537.

Test Plan: ninja check && ninja check-functional

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15538