On iOS make the AVAudioSession.Category configurable to support the silent switch

**Is your feature request related to a problem? Please describe.**
When playing audio with iOS, users expect for apps to silence the audio when the physical silent switch is toggled off.

This is supported in the API by setting the appropriate category on AVAudioSession.sharedInstance().setCategory which is hardcoded to "playback" in the plugin.

**Describe the solution you'd like**
Add a configuration parameter for iOS, perhaps in preloadComplex to set the iOS Audio Session category

**Describe alternatives you've considered**
Remove AVAudioSession.sharedInstance().setCategory from the plugin and let each app set it's own category in AppDelegate.

**Additional context**
See documentation here: https://developer.apple.com/documentation/avfaudio/avaudiosessioncategory