feat: extract AudioContext from audio_context_config (#1440)

# Description

To properly extend the `audioplayers_platform_interface` it is necessary
to expose some of their classes. To do this in a further step, it is
cleaner to extract the `AudioContext` from the `audio_context_config`
and only expose the `AudioContext` and not the `AudioContextConfig` in
the platform interface.

## Migration instructions

Before:
```
import 'package:audioplayers_platform_interface/api/audio_context_config.dart';
```

After:
```
import 'package:audioplayers_platform_interface/api/audio_context.dart';
import 'package:audioplayers_platform_interface/api/audio_context_config.dart';
```

Author: Gustl22

getting_started.md

@@ -196,8 +196,8 @@ While each platform has its own set of configurations, they are somewhat related
 It provides generic abstractions that convey intent, that are then converted to platform specific configurations.
 
 Note that if this process is not perfect, you can create your configuration from scratch by providing exact details for each platform via
-[AudioContextAndroid](https://pub.dev/documentation/audioplayers_platform_interface/latest/api_audio_context_config/AudioContextAndroid-class.html) and 
-[AudioContextIOS](https://pub.dev/documentation/audioplayers_platform_interface/latest/api_audio_context_config/AudioContextIOS-class.html).
+[AudioContextAndroid](https://pub.dev/documentation/audioplayers_platform_interface/latest/api_audio_context/AudioContextAndroid-class.html) and 
+[AudioContextIOS](https://pub.dev/documentation/audioplayers_platform_interface/latest/api_audio_context/AudioContextIOS-class.html).
 
 ```dart
   player.setAudioContext(AudioContext(

packages/audioplayers/lib/audioplayers.dart

@@ -1,3 +1,4 @@
+export 'package:audioplayers_platform_interface/api/audio_context.dart';
 export 'package:audioplayers_platform_interface/api/audio_context_config.dart';
 export 'package:audioplayers_platform_interface/api/for_player.dart';
 export 'package:audioplayers_platform_interface/api/log_level.dart';