FEATURE: add after-reviewable-post-user plugin outlet (#9397)

FEATURE: add after-reviewable-post-user plugin outlet

Add a plugin outlet after reviewable post user

Add a basic user serializer that includes custom fields.
Allows review queue serializer to include custom fields for its users

app/serializers/user_with_custom_fields_serializer.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+# A basic user serializer, with custom fields
+class UserWithCustomFieldsSerializer < BasicUserSerializer
+  attributes :custom_fields
+
+  def custom_fields
+    fields = custom_field_keys
+
+    if fields.present?
+      if object.custom_fields_preloaded?
+        {}.tap { |h| fields.each { |f| h[f] = object.custom_fields[f] } }
+      else
+        User.custom_fields_for_ids([object.id], fields)[object.id] || {}
+      end
+    else
+      {}
+    end
+  end
+
+  private
+
+  def custom_field_keys
+    # Can be extended by other serializers
+    User.whitelisted_user_custom_fields(scope)
+  end
+end