Output message as json

Hi, I started to use this package in conjuction with `logback-logstash-encoder` for message layout.
Then I noticed that message is output being quoted as string even if it is actually json.

I suppose this is because `EcsJsonSerializer.serializeFormattedMessage` is always used for any message layout in `EcsEncorder`.
https://github.com/elastic/ecs-logging-java/blob/main/logback-ecs-encoder/src/main/java/co/elastic/logging/logback/EcsEncoder.java#L149

Currently is there any way to output message in json or do you have any plan to support such feature?