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.