Add friends functionality.

[theLee3]

This PR adds all friends functionality on iOS, macOS and Android.

It also includes some code cleanup and method refactoring that didn't seem worth splitting into another PR given how many changes would be shared between the two. The primary change being that async method calls are passed through other methods where possible as opposed to using try/await within the calling method. This provides some performance improvements.

This is submitted as a draft as more testing is needed on the Friends.access getter on Android as explained below.

[theLee3]

On Android, my calls to Friends.access behave unexpectedly. I am uncertain if it is a device/account issue or something else, so I am asking others to test the behavior out and report back.

Logs have been added on the Android platform codebase for help in diagnosing. In my tests, the cached Player object returned by getCurrentPlayer always has the following CurrentPlayerInfo value, despite actually having friends list access.

zza{FriendsListVisibilityStatus=0} // unknown

The method is written to retry, passing forceRefresh to getCurrentPlayer when this status is returned, as per the docs. However, refreshing the current Player always leads to a CurrentPlayerInfo value of null. Very strange...

Again, the friends list access request went through, and I can load friends despite this.

Also of note, if I go into my privacy settings on my Play Games player profile and attempt to toggle the friends list access for any individual app, an error occurs. I have toggled profile privacy, cleared caches & data for my app, play games services, the play store, and the play games app. Nothing has yet resolved the issue.

[theLee3]

To test, update the following in pubspec.yaml:

games_services:
  git:
    url: https://github.com/Abedalkareem/games_services.git
    ref: feature/friends
    path: games_services