add json extractor that writes all found fields into message (json k=v)

[jalogisch]

Problem description

When you try to replace the json extractor that just takes takes all fields and write them to the message that is currently not possible.

The only option you have is to extract known fields from json but you are not able to explore your data or have dynamic messages with changing content.

Environment

• Graylog Version: 2.2.2