Add sorting_column to Field

[goosys]

I've split the Sortable from #2658.

Usage

• It is possible to use a name as attribute_name that doesn't exist in the DB column but is defined in the model.

class OrderDashboard < Administrate::BaseDashboard
  ATTRIBUTE_TYPES = {
    full_address: Field::String.with_options(
      getter: ->(field) {
        r = field.resource
        [
          r.address_line_one,
          r.address_line_two,
          r.address_city,
          r.address_state,
          r.address_zip
        ].compact.join("\n")
      },
      sorting_column: :address_zip
    ),

• Attributes that cannot be sorted will not have a sort link in the table header.

module Administrate
  module Field
    class Password < Field::Base
      def self.sortable?
        false
      end

class CustomerDashboard < Administrate::BaseDashboard
  ATTRIBUTE_TYPES = {
    # Since the aggregation is done on the Ruby side and sorting is not possible, it is set to not be sortable.
    lifetime_value: Field::Number.with_options(prefix: "$", decimals: 2, sortable: false),

[goosys]

I rebased after #2658 was merged and made a few additional fixes.

[goosys]

I feel that we need to discuss the option name.
We already have sort_by and order options related to sorting, but the newly added option doesn’t fully serve the same purpose as either of these, though there is some overlap.
For now, I’ve named it sorting_column to avoid conflicts, but it doesn’t feel very elegant. I’d appreciate any ideas you might have.