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.

log4j2-ecs-layout の EcsLayout クラスから始め、サービス名と設定値から event.dataset がどのように設定されるかを追跡してください。ContextData/MDC がどのようにシリアライズされるか、またそこで event.dataset を設定すると重複フィールドが生成される理由を確認してください。重複出力なしでサポートされたイベント単位の上書きを定義して実装し、1つのログファイル内で複数のイベントデータセットを扱うカバレッジを追加できれば完了です。

索引モデルが issue の本文から書いたものです。