Support "add only" SelectFields

[stephenh]

Something like AddSelectField (?) but for this use case:

https://github.com/homebound-team/internal-frontend/pull/4008/files?w=1#r1258480223

Where we want a SelectField that doesn't have a value prop, b/c every time the onSelect is called, we invoke a side-effect, but then revert to an empty select field.