Add annotation for default plugin values.

  - Use this to specify the string value you'd want as the default fallback value for a @PluginAttribute, @PluginValue, or @SensitivePluginValue.
  - Part of LOG4J2-652.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1597360 13f79535-47bb-0310-9956-ffa450edef68