Support structured logging with Logback

As logback user, I'd like to have the ability to create structured logs as https://github.com/elastic/ecs-logging-java/tree/master/log4j2-ecs-layout#structured-logging

Currently the community relies on "net.logstash.logback.encoder.LogstashEncoder" to generate structured log statements which leverages `Markers`. Although not ideal, as Markers are intended for filtering: http://logback.qos.ch/manual/filters.html, I think it would be useful if the EcsEncoder could support this scenario without requiring an additional dependency library

Start by reading EcsEncoder and the linked ECS structured-logging example, then review how the current LogstashEncoder and Markers approach represents structured fields. Clarify the supported structured-log behavior and acceptance criteria before implementation; done should mean EcsEncoder supports the agreed scenario without the additional dependency.

Written by the indexing model from the issue text.