feat: Add sd_bus_message_get_cookie (#490)

[howey]

Add sd_bus_message_get_cookie() and sd_bus_message_get_reply_cookie() as Message::getCookie() and Message::getReplyCookie().

[sangelovic]

I'm also thinking about a new (integration) test case verifying this feature and showing how the serial number in a method call message would match a serial number in the reply message. Not sure how familiar are you with the tests... I may help you here by writing it and pushing to your branch.

[howey]

Sure, go ahead. This is roughly my code

  auto callback = [](sdbus::MethodReply reply, std::optional<sdbus::Error> error) {
    auto n = reply.getReplyCookie();
    std::cerr << "reply cookie " << n << std::endl;
  };

  for (int i = 0; i < 3; i++) {
    auto method = proxy->createMethodCall(interfaceName, methodName);
    proxy->callMethodAsync(method, callback);

    auto n = method.getCookie();
    std::cerr << "method cookie " << n << std::endl;
  }

[sangelovic]

OK thanks, will do tomorrow.

[sangelovic]

Sorry, took me longer than anticipated, busy with other things :-) But now the test is there, the CI is passing, so let's go ;-)