Previously, we could only use the default widget for each field in a tool

plugin's options. This change allows us to specify which widget we want to use

on a field, along with any widget-specific attributes.
I also added a check to make sure that the field class we get is an actual

Field class.

Made fields without widgets specified.
Made a field, specifying the widget but no attributes, and saw the widget's

  default attributes get used.
Made a field, specifying the widget and its attributes.

I saw that the above fields were displayed correctly on the "Change Review Bot

Tool" page (see the attached screenshot), and was able to modify them.

Tried to make an invalid field, and saw the appropriate exception.
Tried to make an invalid widget, and saw the appropriate exception.

I also checked that all existing tool plugin forms display properly.