Make new sensor interface internal for now (#4811)

com.unity.ml-agents/Runtime/Sensors/BufferSensor.cs

 
 namespace Unity.MLAgents.Sensors
 {
-    public class BufferSensor : ISensor, IDimensionPropertiesSensor
+    internal class BufferSensor : ISensor, IDimensionPropertiesSensor
     {
         private int m_MaxNumObs;
         private int m_ObsSize;

com.unity.ml-agents/Runtime/Sensors/BufferSensorComponent.cs

     /// A component for BufferSensor.
     /// </summary>
     [AddComponentMenu("ML Agents/Buffer Sensor", (int)MenuGroup.Sensors)]
-    public class BufferSensorComponent : SensorComponent
+    internal class BufferSensorComponent : SensorComponent
     {
         public int ObservableSize;
         public int MaxNumObservables;

com.unity.ml-agents/Runtime/Sensors/IDimensionPropertiesSensor.cs

     /// The Dimension property flags of the observations
     /// </summary>
     [System.Flags]
-    public enum DimensionProperty
+    internal enum DimensionProperty
     {
         /// <summary>
         /// No properties specified.
     /// <summary>
     /// Sensor interface for sensors with special dimension properties.
     /// </summary>
-    public interface IDimensionPropertiesSensor
+    internal interface IDimensionPropertiesSensor
     {
         /// <summary>
         /// Returns the array containing the properties of each dimensions of the