Support defining maximum bandwidths at diferent levels.

- Individually for each backend.
- For the proxy client (i.e. signaling server using cluster of proxies).
- For the proxy server / MCU.

The smallest bandwidth configured will be used, e.g. if a backend has a
larger limit than the MCU assigned for the stream, the limit of the MCU
server will be used.

api_proxy.go

@@ -174,6 +174,7 @@ type CommandProxyClientMessage struct {
 	StreamType  string `json:"streamType,omitempty"`
 	PublisherId string `json:"publisherId,omitempty"`
 	ClientId    string `json:"clientId,omitempty"`
+	Bitrate     int    `json:"bitrate,omitempty"`
 }
 
 func (m *CommandProxyClientMessage) CheckValid() error {