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.

[andsel]

The only field added is the one the user selects with source_ip_fieldname which is host by default. The plugin extract the source IP and set that field is not yet present:

logstash-input-udp/lib/logstash/inputs/udp.rb

Line 177 in b0833dd

event.set(source_ip_fieldname, ip_address) if event.get(source_ip_fieldname).nil?

If the user customize the name is out of control of ECS but if the user leverage the default behaviour and has ecs_compatibility enabled we should switch that field to a new name [host][ip]. This rename could happen at register time