519db4d951 add Network::to_core_arg() method (connormullett)
Pull request description:
closes: #1207
Adds converting `Network` to its `bitcoind` equivalent.
The arguments for -chain can be found in the documentation and is one of the following:
main, test, signet, regtest
ACKs for top commit:
apoelstra:
ACK 519db4d951
Kixunil:
ACK 519db4d951
Tree-SHA512: 5fd805d654f7c30f87ff877fe90e19490d0deb73b46ce87cc6b43d30595eb9d2de3f646f58a5d72180c3e8cc6a9b614bfe6753ecd6c21b8d193a8d862e3f887f