Supporting global illumination for macOS/metal

## Desired behavior

Currently, global illumination works on Ubuntu but not on macOS. It would be great for GI to work on macOS though.

## Additional context

For https://github.com/gazebosim/gz-sim/pull/2550, when the homebrew CI build runs a test with GI enabled, it crashes on macOS. Output below:

```
281: [ RUN ] CameraSensorGlobalIlluminationTest.GlobalIlluminationEnabled
281: libc++abi: terminating with uncaught exception of type Ogre::FileNotFoundException: OGRE EXCEPTION(6:FileNotFoundException): Cannot locate resource CrossPlatformSettings_piece_all.metal in resource group General or any other group. in ResourceGroupManager::openResource at /tmp/ogre2.3-20230918-86352-14va4s/ogre-next-2.3.1/OgreMain/src/OgreResourceGroupManager.cpp (line 793)
```

I also noticed that the [GI unit test](https://github.com/gazebosim/gz-rendering/blob/gz-rendering8/test/common_test/GlobalIllumination_TEST.cc) also skips for Apple platform currently too.