Add scorecard support on custom field update API

Whenever you modify a custom, scorecard-type field to point it to a different scorecard, all previously entered values in this field across Aha are lost. This makes sense, as the structure of the new scorecard may be completely different from the old one.

In order to alleviate the pain for teams who need to modify a scorecard's structure, an easy workaround would be to use the APIs and "migrate" values in bulk from the old scorecards to the new ones (according to whatever calculation is appropriate).

However, the current PUT /api/v1/features/:id  API does not allow the update of a custom field if it is of type scorecard. Would it be possible to support this in the future?