Issue 663- Add custom attribute

[gracelombardi]

Still a WIP. Getting a vendor attribute has no value type 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 309, in read
    self.request_payload.read(tstream, kmip_version=kmip_version)
  File "/usr/local/lib/python3.8/dist-packages/PyKMIP-0.11.0.dev1-py3.8.egg/kmip/core/messages/payloads/register.py", line 210, in read
    self._template_attribute.read(
  File "/usr/local/lib/python3.8/dist-packages/PyKMIP-0.11.0.dev1-py3.8.egg/kmip/core/objects.py", line 3478, in read
    attribute.read(tstream, kmip_version=kmip_version)
  File "/usr/local/lib/python3.8/dist-packages/PyKMIP-0.11.0.dev1-py3.8.egg/kmip/core/objects.py", line 117, in read
    raise Exception("No value type for {}".format(enum_name))
Exception: No value type for VENDOR_ATTRIBUTE