BADGER-130: Test CDN failover (extended plugin functionality)

[JoeBBC]

📺 What

Extends existing plugin functionality, allowing functional plugins, and adds a global plugin context system.

This is supporting work for tv-client#BADGER-130-InvokeCdnFailover

🛠 How

• Allows registerPlugin to be passed a function
  • Functional plugins are fully compatible with the existing 'event plugin' objects
  • Either or can be used, or combined by defining methods on the functional plugin
• Functional plugins are passed the 'plugin context' & an onContextUpdated callback
• A global 'plugin context' state is maintained to selectively expose state & functionality to plugins

✅ Testing

Integration tests have been implemented in tv-client#BADGER-130-InvokeCdnFailover