Factor BroadcastTransaction out of sendrawtransaction
Summary:
Factor out a new BroadcastTransaction function, performing the core work of the sendrawtransaction rpc, so that it can be used from the GUI code. Move it from src/rpc/ to src/node/.
Partial backport of core PR14978:
https://github.com/bitcoin/bitcoin/pull/14978/commits/81cd9588484cb4f4050ea4e239da0681111795db
Test Plan:
ninja all check ./test/functional/test_runner.py rpc_rawtransaction
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5706