Migrates to Federated Plugin Architecture

[cybex-dev]

This PR migrates the plugin to the federated plugin architecture.

This involves:

• Exposing the TwilioVoicePlatform class.
• Updating the example app to use the new platform interface.
• Adding a breaking change notice to the changelog, as it requires a small code change for existing users, switching from TwilioVoice.instance to TwilioVoicePlatform.instance.
• Fixes an issue where the Twilio Device does not unregister properly on the web platform.