Support for ``EXTRA_RESPONSE_FIELDS``

It is now possible to configure a list of additonal document fields that
should be provided with POST responses. Normally only automatically
handled fields (``ID_FIELD``, ``LAST_UPDATED``, ``DATE_CREATED``,
``etag``) are included in POST payloads.

``EXTRA_RESPONSE_FIELDS`` is a global setting that will apply to all
resource endpoint . Defaults to ``[]``, effectively disabling the
feature.

``extra_response_fields`` is a local resource setting and will override
``EXTRA_RESPONSE_FIELDS`` when present.