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.

[kares]

ECS support here is effectively a NO-OP, we only set one (@metadata) field directly, otherwise field names will depend on the codec used. default codec is JSON, for better ECS compliance of user data, from the input, the plugin needs target name-space support in the codec.

[kares]

we should still consider mentioning ECS in the docs, once JSON codec supports the target => ... namespace