Update PostAggregator to be backwards compat (apache#12138)

This change mimics what was done in PR apache#11917 to
fix the incompatibilities produced by apache#11713. apache#11917
fixed it with AggregatorFactory by creating default
methods to allow for extensions built against old
jars to still work.  This does the same for PostAggregator

(cherry picked from commit eb0bae4)

processing/src/main/java/org/apache/druid/query/aggregation/PostAggregator.java

@@ -23,6 +23,8 @@
 import org.apache.druid.java.util.common.Cacheable;
 import org.apache.druid.segment.ColumnInspector;
 import org.apache.druid.segment.column.ColumnType;
+import org.apache.druid.segment.column.ColumnTypeFactory;
+import org.apache.druid.segment.column.ValueType;
 
 import javax.annotation.Nullable;
 import java.util.Comparator;
@@ -48,10 +50,27 @@ public interface PostAggregator extends Cacheable
   /**
    * Return the output type of a row processed with this post aggregator. Refer to the {@link ColumnType} javadocs
    * for details on the implications of choosing a type.
+   *
    * @param signature
    */
   @Nullable
-  ColumnType getType(ColumnInspector signature);
+  default ColumnType getType(ColumnInspector signature)
+  {
+    return ColumnTypeFactory.ofValueType(getType());
+  }
+
+  /**
+   * This method is deprecated and will be removed soon. Use {@link #getType(ColumnInspector)} instead. Do not call this
+   * method, it will likely produce incorrect results, it exists for backwards compatibility.
+   */
+  @Deprecated
+  default ValueType getType()
+  {
+    throw new UnsupportedOperationException(
+        "Do not call or implement this method, it is deprecated, use 'getType(ColumnInspector)' instead"
+    );
+  }
+
 
   /**
    * Allows returning an enriched post aggregator, built from contextual information available from the given map of