make object count labeler's idlabelconfig field settable

com.unity.perception/Runtime/GroundTruth/Labelers/ObjectCountLabeler.cs

         /// <summary>
         /// The <see cref="IdLabelConfig"/> which associates objects with labels.
         /// </summary>
-        public IdLabelConfig labelConfig => m_LabelConfig;
+        public IdLabelConfig labelConfig
+        {
+            get => m_LabelConfig;
+            set => m_LabelConfig = value;
+        }
 
         /// <summary>
         /// Fired when the object counts are computed for a frame.

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