Sign a message with the private key of an address.
privkeystringREQUIREDmessagestringREQUIREDcurl --user myuser --data-binary '{"jsonrpc":"1.0","id":"1","method":"signmessagewithprivkey","params":["privkey","my message"]}' -H 'content-type:text/plain;' http://127.0.0.1:8332/"H3/9bEPGhd0THi...base64_signature..."