Add attributes to give classes to choices label, legend and fieldset on multipleRadio and multipleCheckbox

In need to style the choices label, I added the choicesClasses to the widgets attributes, so we can style the choice label.
To style the legend tag inside the fieldset, I used the cssClasses.label attribute. Maintaining the label function for the field.
Finally, I wanted to style the fieldset tag, so I added the fieldsetClasses attribute to the field.