Enable frozen string for external auth

Partially addresses:

https://gitlab.com/gitlab-org/gitlab/issues/27703

spec/lib/gitlab/external_authorization/access_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::ExternalAuthorization::Access, :clean_gitlab_redis_cache do

spec/lib/gitlab/external_authorization/cache_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::ExternalAuthorization::Cache, :clean_gitlab_redis_cache do

spec/lib/gitlab/external_authorization/client_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::ExternalAuthorization::Client do

spec/lib/gitlab/external_authorization/logger_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::ExternalAuthorization::Logger do

spec/lib/gitlab/external_authorization/response_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::ExternalAuthorization::Response do