filter-codec idea

@jsvd suggested some time ago to create a filter that could leverage codecs to do the filtering which would have the advantage of avoiding dual codec/filter implementations like the json codec and filter - we could have only the json codec and leverage the json codec into a filter-codec to create a json filter.

This issue is to keep a trace & followup on that idea.

Also, the same has been suggested for xml in https://github.com/logstash-plugins/logstash-filter-xml/issues/22