RaindropBootstrapper can launch in headless mode

restore this broken functionality, since that time we refactored the raindrop bootstrapper and broke it.

Assets/Raindrop/Bootstrap/BootstrapScene.unity

@@ -152,6 +152,7 @@ MonoBehaviour:
   m_Script: {fileID: 11500000, guid: fa61eabca29ec274e822d94f50702a99, type: 3}
   m_Name: 
   m_EditorClassIdentifier: 
+  startUI: 1
 --- !u!4 &278446682
 Transform:
   m_ObjectHideFlags: 0

Assets/Raindrop/Bootstrap/RaindropBootstrapper.cs

@@ -17,11 +17,16 @@ namespace Raindrop.Services.Bootstrap
     //Just attach this script as a component of the game manager. You don't need anything else, unless you want the UI.
     public class RaindropBootstrapper : MonoBehaviour
     {
+        [SerializeField] public bool startUI = false;
+        
         private void Awake()
         {
             Start_Raindrop_CoreDependencies();
 
-            StartUIScene();
+            if (startUI)
+            {
+                StartUIScene();
+            }
         }
 
         private void StartUIScene()