[rcore] [GLFW] [SDL] Add `GetGamepadGUID()

[ghost]

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[zet23t]

So the purpose of this change is to provide a way to uniquely identify controllers and using a GUID, but most platforms seem to not support getting this information? At least it doesn't seem like JavaScript allows it and for Android I couldn't find information either. I looked up if Unity supports controller GUIDs, but I could not find much about this either.

If the only platform this will work on is SDL, does it really make sense to give a raylib user the false sense that using this function is going to be useful?