This adds the API call to return all fields for a single layer3domain.
This should serve as a nice basis for other parts to be implemented.
All fields are put together into a single json document, merged together
with the other attributes of the table and then returned to the
requester.
For this to work, I have added a new function that takes a list of
key looking things and converts them into json.
At the same time, it also can convert json looking payloads and
prepare it for the database (that last part was not intended, but
works).
With the many columns where setting attributes is possible, this
functionality should help quite a bit.
