[Chronik] Add load_tx

Summary:
Load transactions from the disk using file_num, data_pos and undo_pos, which we already indexed.

Allows us to read txs from the node when requested by the user.

Depends on D13428.

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13464