Manual update for users and beta groups

[lvalenta]

This MR fixes issues in Apple's API Spec:

• Added missing user role enum case: GENERATE_INDIVIDUAL_KEYS
• Updated BetaGroup EP to return array of BetaGroups and not BetaTesters
• Updated BetaTesters EP to return array of BetaTesters and not Builds.

That may not be all of those issues, just the ones I encountered.

[MortenGregersen]

There are more responses with wrong data type. All except the AppCategory ones. I have made the same changes in my library: https://github.com/MortenGregersen/Bagbutik/pull/147/files
If you haven't already, please file feedback to Apple and duplicate my feedback (FB13540097).

Regarding the user role, I have made the same change.
https://github.com/MortenGregersen/Bagbutik/pull/149/files
Again, if you haven't already, please file feedback to Apple and duplicate my feedback (FB13546172).

[AvdLee]

Looks good, thanks for your contribution!