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!

Comienza localizando la integración de Unity Game Center Access Point y comprobando la iOS Game Center API subyacente para conocer las opciones de posicionamiento compatibles. Se considera completado cuando el Access Point puede colocarse en cualquier posición de la pantalla, o cuando se documenta la limitación de la API si esto no es compatible.

Escrito por el modelo de indexación a partir del texto del issue.