Payment Plugin: Google Iap

1. Setup API Console project: https://developers.google.com/android-publisher/authorization

   When create Oauth 2.0 Client ID specify redirect URI: https://example.com/amember/payment/google-iap/redirect

2. Specify Client ID and Client Secret in plugin configuration page, and click save.

3. Click on "Generate Refresh token" link and authorize access.

4. Create Real-time Developer notifications

   configure push subscriptions to this endpoint:

   https://example.com/amember/payment/google-iap/ipn