Allow Game Center Access Point to be placed anywhere on screen

**Is your feature request related to a problem? Please describe.**
Right now, the Game Center Access Point can only be placed in four pre-defined locations in the corners of the screen. This constrains UI design as we have to work around these locations instead of integrating it better into our UI design.

**Describe the solution you'd like**
I would like to be able to place the access point at any given position.

**Describe alternatives you've considered**
Since the Access Point is provided by the OS, I'm not sure there is an alternative.

**Additional context**
We like the functionality of the access point, we just want it to be easier to work with!

Beginne damit, die Unity Game Center Access Point-Integration zu finden und die zugrunde liegende iOS Game Center API auf unterstützte Positionierungsoptionen zu prüfen. Als erledigt gilt die Aufgabe, wenn der Access Point an einer beliebigen Bildschirmposition platziert werden kann oder die API-Einschränkung dokumentiert ist, falls dies nicht unterstützt wird.

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