[chronik] add blockchain.transaction.broadcast electrum method

Summary: A rpc to broadcast a raw hex transaction. No checks of any kind are done before broadcasting (tx may burn funds or tokens).

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, tobias_ruck, Fabien

Reviewed By: #bitcoin_abc, tobias_ruck, Fabien

Subscribers: tobias_ruck, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17370