Implement ECS-Compatibility Mode

[yaauie]

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 Filter populates fields verbatim from the first event received when
autodetect_column_names => true, it may benefit from requiring a target
namespace to be provided when run in ECS-Compatibility mode with this option.