Implement ECS-Compatibility Mode

[kares]

As a part of the effort to make plugins able to run in an ECS-Compatible manner by default in an upcoming release of Logstash, this plugin needs to either implement an ECS-Compatibility mode or certify that it does not implicitly use fields that conflict with ECS.

Since this Codec produces events verbatim from the encoded structure of the input it is given, it may benefit from requiring a target => ... namespace to be provided when run in ECS-Compatibility mode.