Expose playerIndex / DeviceID in GameController framework for Wwise Motion support in Unity

**Is your feature request related to a problem? Please describe.**
We cannot use Wwise Motion on Apple platforms for haptics due to missing `playerIndex` / `DeviceID` exposure in the Apple Unity Plug-ins.

See Wwise Motion [documentation](https://www.audiokinetic.com/en/public-library/2024.1.9_8920/?source=SDK&id=integrating_elements_motion.html) that requires for all Apple platforms to set the `DeviceID` even for single player games. Otherwise Wwise Motion is not playing any haptics / rumble on Apple platforms.

We use it on all other platforms like Windows and consoles. Thus, the need to have that covered to bring also the same experience to Apple platforms.

**Describe the solution you'd like**
Being able to set the `playerIndex` through C# functions / `Get/Set` methods within Unity and then to retrieve the `DeviceID` needed for Wwise Motion.

**Describe alternatives you've considered**
Shipping on Apple platforms without haptics / rumble support.

**Additional context**
Submitted Apple Feedback ID: FB18696265

Start by reading the GameController framework and the Apple Unity Plug-ins documentation linked in the issue, then trace how player input is exposed through C# on Apple platforms. The work is done when Unity callers can set playerIndex and retrieve the DeviceID required by Wwise Motion for Apple haptics.

Written by the indexing model from the issue text.