Allow specifying scope in with_options for a belongs_to field.

Created by: k3rni

Addresses #563 (closed).

It's now possible to do:

class WidgetDashboard < Administrate::Dashboard
  ATTRIBUTE_TYPES = [
    manager: Field::BelongsTo.with_options(class_name: "User", scope: :managers)
  ]
end

where the User model has a scope named managers. If not specified, defaults to all, so no change in behaviour.