[Fleet] Support spaces in fleet object created by Fleet Server

[nchaulet]

Support spaces in fleet object created by Fleet Server

We are going to make Fleet space aware => Fleet server need to populate the namespace accordingly tin the document it manage.

Pass space ID through from:
.fleet-enrollment-api-keys to .fleet-agents documents.
.fleet-actions to .fleet-actions-results documents.

The namespace property has already been added to all of the .fleet* mappings managed in the fleet

Details

.fleet-policies

The namespace should be populated in Kibana base on the saved object namespace.

.fleet-enrollment-api-keys

The namespace should be populated in Kibana based on the related agent policy.

.fleet-agents

The namespace should be populated on enrolment, we should also update the value when assigning a new policy to the agent, this is done in Kibana.

.fleet-actions
We should populate the namespace when creating the action in Kibana


.fleet-actions-results
We should populate the namespace when creating the action result it should be the same as the corresponding .fleet-actions.

Open questions

Space will not work correctly if Fleet server is not updated, how do we communicate that? Cc @kpollich