[contrib] Add generic cli for calling RPC methods

Created by: dagurval

This adds a main section to client.py so it can be used to test any RPC calls.

Test plan:

$ python3 client.py "server.ping"
{'id': 0, 'jsonrpc': '2.0', 'result': None}

$ python3 client.py "blockchain.address.get_first_use"
"bitcoincash:qqfc3lxxylme0w87c5j2wdmsqln6e844xcmsdssvzy"
{'id': 0, 'jsonrpc': '2.0', 'result': {'block_hash':
'000000000000000002a04f56505ef459e1edd21fb3725524116fdaedf3a4d0ab',
'block_height': 597843, 'tx_hash':
'cef9c1485f166e92adf0b5d42e4ad202e4f5f9e2e7a072d0a6864c61c2cd6fa0'}}

$ python3 client.py "blockchain.address.get_scripthash"
"bitcoincash:qqfc3lxxylme0w87c5j2wdmsqln6e844xcmsdssvzy"
--server=bch.imaginary.cash
{'id': 0, 'jsonrpc': '2.0', 'result':
'273139c9327743dccc1030fc8d009e63e41e147c6b0449913d2dea5b71eef230'}