Add a remote config option to disable libsignal transport in prod

Co-authored-by: Sergey Skrobotov <sergey@signal.org>

ts/RemoteConfig.ts

@@ -27,6 +27,7 @@ export type ConfigKeyType =
   | 'desktop.senderKeyMaxAge'
   | 'desktop.experimentalTransportEnabled.alpha'
   | 'desktop.experimentalTransportEnabled.beta'
+  | 'desktop.experimentalTransportEnabled.prod'
   | 'desktop.cdsiViaLibsignal'
   | 'global.attachments.maxBytes'
   | 'global.attachments.maxReceiveBytes'

ts/textsecure/SocketManager.ts

@@ -559,8 +559,14 @@ export class SocketManager extends EventListener {
         : TransportOption.ShadowingLow;
     }
 
-    // in prod, using original
-    return TransportOption.ShadowingLow;
+    // in prod, switch to using 'ShadowingHigh' mode, unless user opts out,
+    // in which case switching to `ShadowingLow`
+    const configValue = window.Signal.RemoteConfig.isEnabled(
+      'desktop.experimentalTransportEnabled.prod'
+    );
+    return configValue
+      ? TransportOption.ShadowingLow
+      : TransportOption.Original;
   }
 
   private connectLibsignalUnauthenticated(): AbortableProcess<IWebSocketResource> {