Expose Subscription Subscription Introductory Offer period

[keesvandieren]

Subscriptions in Google Play and App Store support a trial period, in which users can try-out the app for free or for a reduced price.

This information is not yet exposed to gdxpay.

I am working on a PR for Google Billing, Robovm implementation, but it is also missing for all other implementations

Please select the affected platforms and payment service implementation

• [ ] Amazon
• [X] googlebilling
• [ ] googlepay
• [ ] apple iosmoe
• [X] apple robovm

[MrStahlfelge]

I am not sure if Amazon has such a feature, however, the implementation completely lacks any subscription features. Good to have it for the implementations supporting subscriptions.

[keesvandieren]

Pull request is merged, we can close the issue