Embedded Chapter Support

## 🚀 Feature Requests

Embedded Chapter Support in Audio Files

### Contextualize the feature

Podcasts and Audio Books can have embedded chapters.

Chapters can include a title, playback position, and sometimes embedded artwork.

You can read more about this feature [here](https://auphonic.com/blog/2013/07/03/chapter-marks-and-enhanced-podcasts/).

### Describe the feature

A MediaItem could have a List of Chapters. Each Chapter object could have a Title, Position, and perhaps embedded artwork.

[MNAVChapters](https://github.com/michaelnisi/MNAVChapters) is an iOS example of this functionality.

[ChapterReader](https://github.com/PaulWoitaschek/ChapterReader) is an Android Library that can read chapters.

[FFmpegMediaMetadataRetriever](https://github.com/wseemann/FFmpegMediaMetadataRetriever) already supports reading embedded chapters.

Other platforms probably apply, but I am not familiar enough with them.

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [x] :tv: tvOS
- [x] :monkey: Xamarin.Forms

The issue names no repository files, tests, or entry points; begin by locating the MediaItem model and the platform media readers, then compare the linked chapter-reading libraries. Done means defining chapter metadata and reading embedded chapters consistently on the affected iOS, Android, tvOS, and Xamarin.Forms platforms, with coverage for the supported formats.

Written by the indexing model from the issue text.