[LOG4J2-356] Create a JSON Layout. Refactor a duplicate method from the JSON and XML layouts into Throwables.getThrowableStringList(throwable);

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1514893 13f79535-47bb-0310-9956-ffa450edef68