AFAIK there is no such option for good reason - you can simply add a CheckBox widget with the label "select all" and have the multi select input conditionally disabled. So either the user explicitly selects any of the provided option or they just chose your select all checkbox. There is no need for the multi select input widget to handle that (and it shouldn't - modules / widgets should focus on one very specific use case, and in this case it is about allowing multiple selection similar to a plain HTML multi select field).
