Add an option to select image size when adding an image field to the metadata block

[erral]

This perhaps needs changes both here and in volto-widgets-view.

It would be nice that the metadata block allows selecting the image scale size to be used when rendering this block, this way the user can select the size he wants to use.

The value selected by the user should be passed to volto-widgets-view ImageWidget which in essence renders the image in https://github.com/eea/volto-widgets-view/blob/master/src/components/theme/Widgets/ImageWidget.jsx