Add created_at metadata to saved objects

[cjcenizal]

See #9202

Add a created_at root level field to all SOs, similar to updated_at

We can surface this metadata in the UI to differentiate between objects with the same name.

[pgayvallet]

Reopening as this wasn't completed and discussions around this came back recently

[pgayvallet]

From our slack discussion:

• Add a new root-level created_at field to the SavedObject type
• Field will be automatically populated by the system when an object is created
  • similar to what is done for updated_at
• Field will have to be optional (same as updated_at) because we can't properly populate it for existing objects
• Main concern is regarding what we do with this value for import and copy to space operations
  • for now, we should just preserve the value
  • later on, we can eventually add options to the impacted APIs / UIs to have the user choose the behavior
    • e.g choose between keeping the import file's creation dates or repopulate it for all imported objects during an import

[pgayvallet]

cc @elastic/kibana-core given I'm not sure the team is subscribed to the issue given its age.

[afharo]

Main concern is regarding what we do with this value for import and copy to space operations

++ I'd even say: it may be different when overwriting vs. importing/copying to space.

But I like the plan to just preserve the value for now 😇