Add runtime attribute to detect whether a log file uses a structured formatter

Structured formatters like json-formatter or xml-formatter have a well-defined format of log statements. This allows log files that use such a formatter to be filtered by date or level. HAL-1429 describes such a feature for the management console. 

To implement this, you must know whether a log file uses a structured formatter. This feature request is about adding this information to the runtime attributes of /subsystem=logging/log-file=foo.