ArrayField button improvement

Latest version.

Arrayfield, should be passed also addButtonClassName prop by which you can override the hardcoded values

className={addButtonClassName ?? "align-self-end mt-15"}

addButtonClassName default value undefined.

As it is now, sometimes you can have a weird gap to the top that is impossible to override as it is basically inline style. Plus not having any specific classname to the add button, makes it hard to target specifically array field add buttons, without accidentally catching other buttons that you have in the app by this selector.