Add ability to customize filter policy to auxiliary input fields

The field infrastructure has a "trait" that specifies the valid types for the default active scalar field. Most filters have at most one input field. The majority of the remaining filters use the same types for inputs. There are some examples, however, where some auxiliary fields use different types. This change adds a second template parameter to FieldTraits to customize the valid types of the input scalar field to different fields.

These changes are in anticipation of the removal of list specifier tags on worklet ControlSignature arguments.