Iap plugin (2.5.1.2). How to set custom payload value?

Hi there! In our game project we are using iap plugin and very happy with it. Now we want to extend payment verification functionality. We decided to use developer payload value with our internal order id but we can`t do this. Could developers add api to set the custom payload feature?

@yinjimmy can think about this.

Better if you extend current purchase function just add second parameter. Something like this:
static void purchase(const std::string & name, const std::string & payload);

If payload parameter is empty than use purchase function as before.

no payload because of https://issuetracker.google.com/issues/63381481