[Chronik] Add /raw-tx/:txid endpoint

Summary: Allows querying raw transactions from the mempool or DB, similar to getrawtransaction, except it doesn't need -txindex and also works for the Genesis coinbase.

Test Plan: ninja && ./test/functional/test_runner.py chronik_raw_tx

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13847