Allow billable member sorting by last activity

To support sorting members table by last activity we will allow
the asc/desc sorting of the `last_activity_on` attribute in the REST API

Changelog: changed
EE: true

ee/lib/ee/api/helpers/members_helpers.rb

@@ -9,7 +9,10 @@ module EE
 
         class << self
           def member_sort_options
-            %w(access_level_asc access_level_desc last_joined name_asc name_desc oldest_joined oldest_sign_in recent_sign_in)
+            %w[
+              access_level_asc access_level_desc last_joined name_asc name_desc oldest_joined oldest_sign_in
+              recent_sign_in last_activity_on_asc last_activity_on_desc
+            ]
           end
         end
 

ee/spec/lib/ee/api/helpers/members_helpers_spec.rb

@@ -38,7 +38,12 @@ RSpec.describe EE::API::Helpers::MembersHelpers do
 
   describe '.member_sort_options' do
     it 'lists all keys available in group member view' do
-      expect(described_class.member_sort_options).to match_array(member_sort_options_hash.keys)
+      sort_options = %w[
+        access_level_asc access_level_desc last_joined name_asc name_desc oldest_joined
+        oldest_sign_in recent_sign_in last_activity_on_asc last_activity_on_desc
+      ]
+
+      expect(described_class.member_sort_options).to match_array sort_options
     end
   end
 end