Adding ProxyKmipClient support for the GetAttributeList operation

This change adds support for the GetAttributeList operation to the
ProxyKmipClient. It updates the Pie client API and provides a demo
showing how to use the operation. All relevant test suites are updated
accordingly.

kmip/pie/api.py

@@ -70,6 +70,17 @@ class KmipClient:
         """
         pass
 
+    @abc.abstractmethod
+    def get_attribute_list(self, uid):
+        """
+        Get a list of attribute names for a managed object on a KMIP appliance.
+
+        Args:
+            uid (string): The unique ID of the managed object whose attribute
+                names should be retrieved.
+        """
+        pass
+
     @abc.abstractmethod
     def destroy(self, uid):
         """