WIP: Add sorting_field 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 Order < ApplicationRecord
  def address
    [address_line_two, address_line_one, address_city, address_state, address_zip].compact.join(" ")
  end
end

class OrderDashboard < Administrate::BaseDashboard
  ATTRIBUTE_TYPES = {
    address_line_one: Field::String,
    address_line_two: Field::String,
    address_city: Field::String,
    address_state: Field::String,
    address_zip: Field::String,
    address: Field::String.with_options(
      # Field used for sorting
      sorting_field: :address_zip,
      # Virtual fields cannot be searchable, so 'false' must be specified
      searchable: false
    ),

• 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),