feat: getStorefront method implementation for fetching App Store and Google Play region

[nahlebn1k]

Hello, I've been using this method in my fork for a while. And i think it would be very usefull to have this in main repo.

[hyochan]

@nahlebn1k Hi! Thanks for your contribution.

Before diving into the code review, could you please provide a clear explanation of the purpose of this PR? This will help us avoid any potential misunderstandings and ensure we're all on the same page. Thank you!

[nahlebn1k]

@hyochan Hi! The purpose of this PR is to add the ability to determine which App Store or Play Store region user in. In my case i have different in-app purchases and subscriptions packs for different regions. And in order to offer users specific purchases it's necessary to know their Store region.

In addition Android getBillingConfigAsync() method should be used with existing BillingClient, so, in my opinion, the implementation of this feature suggests itself. Thanks in advance!

https://developer.android.com/reference/com/android/billingclient/api/BillingClient#getBillingConfigAsync(com.android.billingclient.api.GetBillingConfigParams,com.android.billingclient.api.BillingConfigResponseListener)