Issue #661 Added add_attribute operation

[gracelombardi]

Still a WIP. Getting a not implemented error.

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/PyKMIP-0.11.0.dev1-py3.8.egg/kmip/services/server/session.py", line 167, in _handle_message_loop
    request.read(request_data, kmip_version=kmip_version)
  File "/usr/local/lib/python3.8/dist-packages/PyKMIP-0.11.0.dev1-py3.8.egg/kmip/core/messages/messages.py", line 485, in read
    batch_item.read(istream, kmip_version=kmip_version)
  File "/usr/local/lib/python3.8/dist-packages/PyKMIP-0.11.0.dev1-py3.8.egg/kmip/core/messages/messages.py", line 307, in read
    self.request_payload = self.payload_factory.create(
  File "/usr/local/lib/python3.8/dist-packages/PyKMIP-0.11.0.dev1-py3.8.egg/kmip/core/factories/payloads/__init__.py", line 48, in create
    return self._create_add_attribute_payload()
  File "/usr/local/lib/python3.8/dist-packages/PyKMIP-0.11.0.dev1-py3.8.egg/kmip/core/factories/payloads/__init__.py", line 147, in _create_add_attribute_payload
    raise NotImplementedError()
NotImplementedError

[arp102]

This all looks good, I just have one note about some of the function documentation.

I would also encourage you to include more information in the commit messages or pull request description, like what file add_attribute.py is based on, why AddAttribute is useful, and links to the KMIP spec.

I'm also wondering if it would have been easier to try sharing code with another similar payload definition, but if all the other payloads are defined separately, then it makes sense to keep doing that for now.

I don't think this PR fully solves the problem in OpenKMIP#661 though.
For that, we would also need to add support for AddAttribute to the PyKMIP server, not just the client.

[arp102]

Documentation doesn't match code.
Also occurs in some other functions below.