Enable webhook usage through external application

[nicolasbrailo]

The current API doesn't expose a way to parse an ONVIF message from the camera directly, without managing the connection itself. This makes it harder to:

• implement a webhook in a different service, and have reolink_aio API only manage the state of the camera
• have visibility into the state of the camera, and its messages, from an outside service

This commit [PR] introduces a helper class that can parse a Reolink ONVIF message into a somewhat self-descriptive Python dictionary. This helper can be used from reolink_aio itself, or from a 3p application that only needs to parse Reolink ONVIF messages.

This PR also includes an example of how to use reolink_aio to manage camera subscriptions with a webhook to an external server.