Add userinfo oauth endpoint (#32548)

app/controllers/oauth/userinfo_controller.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class Oauth::UserinfoController < Api::BaseController
+  before_action -> { doorkeeper_authorize! :profile }, only: [:show]
+  before_action :require_user!
+
+  def show
+    @account = current_account
+    render json: @account, serializer: OauthUserinfoSerializer
+  end
+end