Shorten the format of properties

Today its format is
 Properties":[{"name":"some_key","value":"some_value"}]

Since "name" and "value" are erroneous the format can be shorten to
 Properties":[{"some_key":"some_value"}]

This pull request is based ontop of https://github.com/majikthys/log4j2-logstash-jsonevent-layout/pull/7
 (it needs be merged afterwards, and only commit https://github.com/finn-no/log4j2-logstash-jsonevent-layout/commit/99ad41750b4866121f50ae5773f721a30e5efca4 needs be evaluated)