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.