Send updated offers to subscribers after publisher renegotiations

When a publisher has a connection the publisher can update the
connection (for example, to add a video track to an audio only
connection) by sending an updated offer to Janus. Janus detects that,
adjusts the connection and then sends back an answer. Once the publisher
connection is updated Janus starts a renegotiation for the subscribers
and generates the offers to be sent to them.

The signaling server did not handle the event, so the offers were not
sent and the subscriber connections were not updated. Now the offers are
sent as needed, which makes possible for the renegotiation to be
completed by the clients.

In this case the "offer" message will also include an "update" parameter
so clients can differentiate between offers to create new connections or
update the existing one.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

api_signaling.go

@@ -647,4 +647,5 @@ type AnswerOfferMessage struct {
 	Type     string                 `json:"type"`
 	RoomType string                 `json:"roomType"`
 	Payload  map[string]interface{} `json:"payload"`
+	Update   bool                   `json:"update,omitempty"`
 }