feat: extract AudioContext from audio_context_config

[Gustl22]

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.

Checklist

• The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, docs:, chore: etc).
• I have read the [Contributor Guide] and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in [example].

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change, but if one directly imports from audioplayers_platform_interface/api instead of audioplayers one will need to change the import statement.

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';

If the PR is breaking, uncomment this header and add instructions for how to migrate from the
currently released version to the new proposed way.

Related Issues

#1352