[autoform] Go through guess_field even when field class is specified using uicfg guess_field does not only find the proper field class for some attribute/relation, but also do some basic configuration depending on e.g. constraints. Before this patch, if one was specifying explicitly the field class using uicfg.autoform_field, this automatic configuration wasn't happening. Change form.field_by_name and ff.guess_field so explicit class are also automatically configured now. Closes #14474840