Allow overriding event.dataset field per event

Currently the EcsLayout class in `log4j2-ecs-layout` defaults the `event.dataset` to the service name + ".log". At most we can override it with a specific value, but that specific value will be written for every log line.

In the project I am working on we are generating multiple kinds of events into a single log file, including access, egress, trace and logs. We want to be able to differentiate these events by the `event.dataset` field as required by an internal observability team but we can't set/override that field per event. If we set it in ContextData/MDC manually it causes the field to be written twice.

I can understand trying to negotiate the event context data per event before writing out standard fields might be opening a can of worms but I'd like to have the conversation as to how situations like this could be handled.

Comience con la clase EcsLayout en log4j2-ecs-layout y rastree cómo se rellena event.dataset a partir del nombre del servicio y del valor configurado. Revise cómo se serializa ContextData/MDC y por qué establecer event.dataset allí produce campos duplicados. Se considera terminado cuando se haya definido e implementado una sobrescritura por evento compatible sin salida duplicada, con cobertura para múltiples datasets de eventos en un único archivo de log.

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