[Feature] add flag to make the full output machine-readable, not only the message

grpcurl --print-output-machine-readable ...
# the output:
{
    "response-headers": [
        {"content-type": "application/grpc"}
    ],
    "http-status-code": 200,
    "response-message": {
        "field1": 123
    },
    "response-message-bytes": "base64string=="
}