Move ActiveRecord::FixtureSet.signed_global_id to this gem

Active Record doesn't depend on GlobalID so this code should not be
defined there.

Instead, we should use the `active_record_fixture_set` `on_load` hook
to define the GlobalID specific method using the GlobalID railtie.

lib/global_id/fixture_set.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class GlobalID
+  module FixtureSet
+    def signed_global_id(fixture_set_name, label, column_type: :integer, **options)
+      identifier = identify(label, column_type)
+      model_name = default_fixture_model_name(fixture_set_name)
+      uri = URI::GID.build([GlobalID.app, model_name, identifier, {}])
+
+      SignedGlobalID.new(uri, **options)
+    end
+  end
+end

lib/global_id/railtie.rb

@@ -37,6 +37,11 @@ class Railtie < Rails::Railtie # :nodoc:
         require 'global_id/identification'
         send :include, GlobalID::Identification
       end
+
+      ActiveSupport.on_load(:active_record_fixture_set) do
+        require 'global_id/fixture_set'
+        send :extend, GlobalID::FixtureSet
+      end
     end
   end
 end