LPS-40873 Add WebRTCClient mailbox and abstract mail

Two things to note here.

First, in `WebRTCConnection`, I added `calling` prefixes to all `WebRTCClient` since we want to emphasize that this attribute is the client that initiated the call (a connection connects two peers). I believe that having only a `_webRTCClient` is ambiguous in this situation.

Now, for the mailbox and mail, I don't know your opinion about public static nested classes, so I'm taking a chance. Tell me if you want them as separate files.