VAST ad support

## 🚀 Feature Requests

It would be really cool if this player can support video ad insertion via VAST.

### Contextualize the feature
This will be useful to include client side ad insertion depending on user metadata. Add the ad breaks with VAST ad tags.

### Describe the feature
Player should be able to support VAST vpaid, js ads so that it can be configured to insert ad breaks into specific time breaks. Such as pre, mid and post roll.

### 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 files, tests, or entry points. Begin by mapping the player integrations for iOS, Android, tvOS, and Xamarin.Forms, then determine how VAST, VPAID, and JavaScript ads could support client-side pre-roll, mid-roll, and post-roll insertion; done would mean a defined design and working support across the selected platforms.

Written by the indexing model from the issue text.