Possibility to set field option as not sortable

Created by: privorotskii

It is possible to set field as searchable, but there is no field option to set it as sortable or not sortable. It could be useful for some cases - not each column at collection page has to be sortable by default. May I create a PR for adding such option? It would be working like this:

  ...
  ATTRIBUTE_TYPES = {
    ...
    comment: Field::Text.with_options(sortable: false),
    ...
  }.freeze
  ...

So the heading of the 'comment' column at the collection page would not be clickable. Or, may be, there is another way to do it I don't know about?