Add payload base classes to prepare for simplifying the client

This change adds payload request and response base classes to
prepare for future simplification updates to the current client
architecture. No new tests are required for this change.

kmip/core/messages/payloads/sign.py

@@ -19,9 +19,10 @@ from kmip.core import attributes
 from kmip.core import enums
 from kmip.core import primitives
 from kmip.core import utils
+from kmip.core.messages.payloads import base
 
 
-class SignRequestPayload(primitives.Struct):
+class SignRequestPayload(base.RequestPayload):
     """
     A request payload for the Sign operation.
 
@@ -51,9 +52,7 @@ class SignRequestPayload(primitives.Struct):
                 managed object will be used instead.
             data (bytes): The data to be signed, in binary form.
         """
-        super(SignRequestPayload, self).__init__(
-            enums.Tags.REQUEST_PAYLOAD
-        )
+        super(SignRequestPayload, self).__init__()
 
         self._unique_identifier = None
         self._cryptographic_parameters = None
@@ -247,7 +246,7 @@ class SignRequestPayload(primitives.Struct):
         })
 
 
-class SignResponsePayload(primitives.Struct):
+class SignResponsePayload(base.ResponsePayload):
     """
     A response payload for the Sign operation.
 
@@ -260,9 +259,7 @@ class SignResponsePayload(primitives.Struct):
     def __init__(self,
                  unique_identifier=None,
                  signature_data=None):
-        super(SignResponsePayload, self).__init__(
-            enums.Tags.RESPONSE_PAYLOAD
-        )
+        super(SignResponsePayload, self).__init__()
 
         self._unique_identifier = None
         self._signature_data = None