[Asset discovery] Remove `entity.*` from the event fields

[chennn1990]

Motivation
As part of the migration to the new Event schemas described here we are aiming to remove the entity field from the top level hierarchy of the event structure, and keep it inside the concrete types (e.g user.entity.*)

Definition of done

• entity is not part of the top level of event structure

Related tasks/epics

• [Entity Store] Support *.entity.id fields on transform pivot kibana#233603
• [Asset Discovery] Migrate to new nested *.entity.* schema #3554

[uri-weisman]

As part of the changes for elastic/kibana#256297, when we remove entity., we should move user fields into the concrete ECS-style field sets (user.), not under entity.user.*.
It means that user.entity.id will have to be placed in user.id.