Make GetAnnotationSpecification() public, by popular demand. (#335)

* Make GetAnnotationSpecification() public, by popular demand.

* Updating changelog

com.unity.perception/CHANGELOG.md

 
 Added a Keypoint Occlusion Override component which allows a user to universally scale all of the keypoint tolerances for a model.
 
+Make IdLabelConfig.GetAnnotationSpecification() public.
+
 ### Changed
 
 Increased color variety in instance segmentation images.

com.unity.perception/Runtime/GroundTruth/Labeling/IdLabelConfig.cs

             m_LabelEntryMatchCache = null;
         }
 
+        /// <summary>
+        /// A structure representing a label entry for writing out to datasets.
+        /// </summary>
-        internal struct LabelEntrySpec
+        public struct LabelEntrySpec
         {
             /// <summary>
             /// The label id prepared for reporting in the annotation
             public string label_name;
         }
 
-        internal LabelEntrySpec[] GetAnnotationSpecification()
+        /// <summary>
+        /// Returns the label entries as structures suited for writing out to JSON datasets.
+        /// </summary>
+        public LabelEntrySpec[] GetAnnotationSpecification()
         {
             return labelEntries.Select((l) => new LabelEntrySpec()
             {