App Premium implementation

[Luc1412]

Summary

This is a work in progress pull request is a first attempt for implementing the upcoming application premium subscriptions available in the app discovery and app profiles currently available in a beta phase for selected apps.

This system is based on the SKU system introduced with the legacy game store. However documentation is just barely available. Just a gist and the legacy SKU docs for the endpoints.

Following steps need to be considers, before this PR is ready for merging:

• [ ] Awaiting clear documentation for the objects to better type the objects
• [ ] Extensive testing. (This is currently just possible for testers with a test SKU, which includes me)
• [ ] Considering if Entitlements should be cached
• [ ] Possible improve structures of methods e.g. instead of Client.entitlements -> SKU.entitlements
• [ ] Possible add shortcuts for fetching entitlements
  • Including: SKU.fetch_entitlements, Guild.fetch_entitlements, User.fetch_entitlements
• [ ] Improve Documentation
• [ ] Format Code with black

Checklist

• [ ] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [x] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)