Implement auto-fill variables for checker option fields

Add an AutoFill attribute to the Field struct that marks option fields
as automatically resolved by the software based on test context, rather
than requiring user input. Auto-fill always overrides any user-provided
value at execution time.

model/form.go

@@ -104,6 +104,11 @@ type Field struct {
 
 	// Description stores an helpfull sentence describing the field.
 	Description string `json:"description,omitempty"`
+
+	// AutoFill indicates the field value is automatically resolved by the
+	// software based on test context. When set, the value should not be
+	// entered by the user.
+	AutoFill string `json:"autoFill,omitempty"`
 }
 
 type FormState struct {