Extend the Event Interface for Plugins with an toJson Method

I would like to extend the Event Interface with an toJson() method which the Event Class already implements. The change would be minimal and a matter of seconds.

Search the Logstash source for the Event Interface and the Event Class implementation of toJson(). Compare their method signatures and inspect the surrounding plugin-facing API. Done means the interface exposes toJson() consistently with the existing Event Class implementation and the relevant project checks pass.

Written by the indexing model from the issue text.