LPS-40873 Add concrete WebRTC mail classes

This adds four WebRTC mail classes. Since they are all the same currently except for the message type they return, perhaps an alternative would be to make WebRTCMail concrete and instanciate one with a given type (a constant static string defined somewhere).
    
On the other hand, I feel like subclassing like it's currently done is better software engineering since adding a new mail type means implementing a new class, not adding a constant to a set of pre-defined strings. All mail classes could be in another package though.
    
This actual string returned by `getMessageType()` is important since it's given as is to the client-side when it asks for new mails in its inbox.
    
Tell me what you think.