Allow user to specify view mode in widget settings for different output scenarios

Problem/Motivation

I have a website setup when different entities/bundles reference one entity.
Due to the different fields available in a form display, I also want to have a different list of fields in the table view.

With the current setup I can use only one view mode which is predefined as a constant (ief_table).

It would be better to have the VIEW mode select field in the field widget settings the same way as we can specify FORM mode, so I can setup any amount of view modes and use them for different field instances.

With this patch, we have a new "Table view mode" field in the field widget settings form and I can choose from different view modes added to the target entity.
By default, we are referencing to OOB view mode machine name (ief_table), so it will not break backward compatibility.