Export webrtc getStats functionality through our API

[atsakiridis]

Turned out to be bigger than anticipated. As part of this issue I'm implementing the first part which is about returning the webrtc stats when disconnecting after normalizing them to json string.

Later we can:

1. Provide specific object to back the json structure and return that to the user for ease of use
2. Provide on demand API to return stats during a call
3. Potentially provide periodic API to return stats every X seconds (see if it makes sense and if API usability is maintained)