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 implement
an ECS-Compatibility mode that does not implicitly use fields that conflict
with ECS.

Absent target_body and target_headers directives, this plugin uses root-level
fields body and headers respectively, which are undefined in ECS and
therefore have the possiblity of conflicting with future versions of ECS. The
plugin may benefit from an ECS-Compatibility mode that requires these directives
to be set explicitly or otherwise reduces the risk of future conflict.