GraphQL: Add 'milestone_due_asc' sort for Issuables

Add GraphQL sorting for milestone_due_asc on an Issuable

        when 'milestone', 'milestone_due_asc'                 then order_milestone_due_asc
        when 'milestone_due_desc'                             then order_milestone_due_desc

in https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/concerns/issuable.rb#L230-247

Will require updating Types::IssuableSortEnum

Due to the complexity of the sort, may require updates to the keyset pagination support