Spatial Audio: figure out what's required to support spatial audio in EffectsPlayer

When a podcast is played through `DefaultPlayer` (eg: streamed with trim silence off) and you have the app set to play spatial audio, the effect works out of the box.

However when you play a podcast through `EffectsPlayer` (eg: downloaded, or trim silence downloaded on the fly content) it doesn't work.

That seems to be what this video is saying as well: https://developer.apple.com/videos/play/wwdc2021/10265/

That support for it is out of the box in `AVPlayer` (the playback engine behind `DefaultPlayer`) and enabled by default, but that you need to do some work to get it to work with `AVAudioEngine` (the playback engine behind `EffectsPlayer`)

Apparently [AVSampleBufferAudioRenderer](https://developer.apple.com/documentation/avfoundation/avsamplebufferaudiorenderer) supports it so that might be one thing to look into. There's an entire sample project for it here:
https://developer.apple.com/documentation/avfaudio/audio_engine/playing_custom_audio_with_your_own_player

Start by comparing the DefaultPlayer path using AVPlayer with the EffectsPlayer path using AVAudioEngine, focusing on why spatial audio works for one but not the other. Read the linked WWDC session and the AVSampleBufferAudioRenderer and custom-player sample documentation. Done means the required implementation approach and scope for spatial audio in EffectsPlayer are established.

Written by the indexing model from the issue text.