iOS: Mixing audio with other apps

# Feature Request
Make this plugin able to mix with background audio from other apps. Support the audio ducking feature as well.

## Motivation Behind Feature
The plugin currently supports only the AVAudioSessionCategories `AVAudioSessionCategoryPlayback` & `AVAudioSessionAmbientSolo`.

This makes it impossible to mix the apps audio with e.g. music coming from spotify in the background.

## Feature Description
The `startPlayingAudio()` should accept an optional parameters `mixWithBackgroundAudio` and `duckBackgroundAudio` for this along the lines of the already accepted `playAudioWhenScreenIsLocked`.

When the parameters to mix with other audio is set to true the AVAudioSessionCategory should be set to

`[self.avSession setCategory:AVAudioSessionCategoryAmbient withOptions:(AVAudioSessionCategoryOptionMixWithOthers, AVAudioSessionCategoryOptionDuckOthers) error:&err];`

(and the audio ducking as well)

## Alternatives or Workarounds
change the AVAudioSessionCategory by hand and install local version of this plugin

Start by locating the iOS implementation of startPlayingAudio() and its AVAudioSession setup. Read how playAudioWhenScreenIsLocked is handled, then trace the existing category and option choices. Done means the optional mixWithBackgroundAudio and duckBackgroundAudio parameters produce the requested background mixing and ducking behavior without changing existing defaults.

Written by the indexing model from the issue text.