PLT-7809: Add support for Kopano Webmeetings WebRTC server (#7590)

* Add support for Kopano Webmeetings WebRTC server

Add an option to select which WebRTC server to use and add support to
use Kopano Webmeetings as backend instead of Janus. If the new
configuration is not set, WebRTC assumes Janus is used as backend.

* Fixup: remove redundant case. default to janus

* Fixup: use GatewayAdminUrl as direct prefix to admin URL entry point

* Fixup: consumeAndClose

model/config.go

@@ -465,6 +465,7 @@ type NativeAppSettings struct {
 
 type WebrtcSettings struct {
 	Enable              *bool
+	GatewayType         *string
 	GatewayWebsocketUrl *string
 	GatewayAdminUrl     *string
 	GatewayAdminSecret  *string