Add userinfo oauth endpoint (#32548)

app/serializers/oauth_metadata_serializer.rb

@@ -2,7 +2,7 @@
 
 class OauthMetadataSerializer < ActiveModel::Serializer
   attributes :issuer, :authorization_endpoint, :token_endpoint,
-             :revocation_endpoint, :scopes_supported,
+             :revocation_endpoint, :userinfo_endpoint, :scopes_supported,
              :response_types_supported, :response_modes_supported,
              :grant_types_supported, :token_endpoint_auth_methods_supported,
              :code_challenge_methods_supported,

app/serializers/oauth_userinfo_serializer.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class OauthUserinfoSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :iss, :sub, :name, :preferred_username, :profile, :picture
+
+  def iss
+    root_url
+  end
+
+  def sub
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
+  def name
+    object.display_name
+  end
+
+  def preferred_username
+    object.username
+  end
+
+  def profile
+    ActivityPub::TagManager.instance.url_for(object)
+  end
+
+  def picture
+    full_asset_url(object.avatar_original_url)
+  end
+end