Implement ECS-Compatibility Mode

[yaauie]

This is a stub issue, and needs to be fleshed out with details specific to
this plugin.

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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
bytes it is given, it may benefit from requiring a target namespace to be
provided when run in ECS-Compatibility mode.