Create a setter for "topLevelLabels"

[Bjoern2]

Hi.
I want to set custom topLevelLabels in co.elastic.logging.logback.EcsEncoder. (E.g. "user.name".)
But there is no setter for this variable.
Could you please add a new setter for this variable? Thanks.
Greetings Björn

[felixbarny]

Would you be interested in creating a PR in https://github.com/elastic/ecs-logging-java/blob/master/logback-ecs-encoder/src/main/java/co/elastic/logging/logback/EcsEncoder.java? I think a addTopLevelLabel method might even be better than a setter.