Enable OAuth PKCE Extension (#31129)

app/lib/oauth_pre_authorization_extension.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module OauthPreAuthorizationExtension
+  extend ActiveSupport::Concern
+
+  included do
+    validate :code_challenge_method_s256, error: Doorkeeper::Errors::InvalidCodeChallengeMethod
+  end
+
+  def validate_code_challenge_method_s256
+    code_challenge.blank? || code_challenge_method == 'S256'
+  end
+end