Pass the thread to the Page inbox with the Handover Protocol

#### Is your feature request related to a problem? Please describe.
Currently, we call the Persona API to _visually_ pass the thread to a dedicated agent but we don't actually leverage the Handover Protocol to switch to a different application.

#### Describe the solution you'd like
We want to pass the thread to the Page inbox to enable the user to live chat with the customer support agent, using the Handover Protocol:

#### Documentation
- https://developers.facebook.com/docs/messenger-platform/handover-protocol
- https://blog.messengerdevelopers.com/tutorial-adding-live-chat-via-the-page-inbox-with-the-handover-protocol-aea2ede75fd