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]

the input is setting 3 [@metadata] fields, these could be improved to follow the [input][rabbitmq] convention prefix
but in terms of ECS no conflicts for the input.

output needs no work.