copy entity to user & hosts structs

[chennn1990]

Summary of your changes

Asset Inventory change:
create a nested hierarchy inside User & Host so the entity will be appeared also there instead of a sibling.

Related Issues

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

Checklist

• I have added tests that prove my fix is effective or that my feature works

[mergify]

This pull request does not have a backport label. Could you fix it @chennn1990? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[uri-weisman]

LGTM 🚀 A few notes:

1. Let’s validate this on GCP and Azure before merging.
2. We should open a ticket to remove the redundant entity.* fields for host and user entities once we pivot to user|host.entity.id. This can be tracked as an action item under elastic/kibana#233603.
3. There are currently no mappings for these fields. we’ll need to add them in the integrations repo:
4. Please don't backport to 9.3

[uri-weisman]

Not necessary related to your changes, but I wonder why we don't use the inventory.WithHost utility here.