[Lens] [Event annotations] Addressing technical debt

[mbondyra]

1. Review functional tests (maybe there's nothing to add, but when developing, we cut some corners here)
2. An expression event_annotation_result runs some inner expressions twice -> one to get fetched annotation datatable, the other to return config layers. Maybe we could rethink it a bit?
3. In the event_annotation_group we have a param annotations[] that takes either manual_point_event_annotation, manual_range_event_annotation or query_point_event_annotation. In the future we're planning on adding new types (for example ML annotation) so maybe it would be better to abstract all the different annotations expression types away the same way as essaggs takes agg_type that can take different forms (date_histogram, count and many more)

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)