Add Subscriptions Functions

[gpene]

Add the following missing functions:

• Ability to Get / List subscription purchases (as documented here)
• Apple Subscription Validation
• Google Subscription Validation

[nikhilbhatia08]

Can I work on this issue ?
I am willing to work on this issue

[gpene]

Hi @nikhilbhatia08 - of course, we're always happy to have community contributions. Feel free to pick this up, and let us know any questions along the way :)

[nikhilbhatia08]

Are these the requirements of this issue :

1. We need to get the list of users who have subscription purchases.
2. We need to validate the subscription of a user using an Apple device.
3. We need to validate a user's subscription using a Google account.
   Is this what this issue means @gpene

[gpene]

It would be:

1. Get a subscription by its corresponding product id.
2. List all the subscriptions of a particular user.
3. Validata a subscription for a user via Apple App Store.
4. Validate a subscription for a user via Google Play Store.

[nikhilbhatia08]

@gpene Yes I will work on them, Thank you.

[nikhilbhatia08]

@gpene Once can you check whether I did the correct thing in validating a subscription via Apple app store and Google play store ? - #182
Once can you check the commit and is it the correct way ?