net/webrtc/signaller: unify webrtcsrc-signaller behavior with gstwebrtc-api

When starting a webrtcsrc-signaller client in Listener mode, only the producers started after the client connection are advertised. There is no way to get the list of already running producers which compels end-users to implement the signaller protocol just to get the list of active producers.

With the gstwebrtc-api, when started in Listener mode, already running producers are advertised at connection time using the same events mechanism as the "producer-added" used to advertise new producers after connection.

This merge request proposes to do the same for the native webrtcsrc-signaller. This way it is easy for the end-user to implement a consumer pipeline that will select a producer automatically based on its meta data for example.

This MR also proposes a new read-only property in the webrtcsrc-signaller to give access to the client ID of the signaller. This "client-id" property can be used to connect consumers to a producer without needing to look in the signalling server logs.