[Fleet] Instructions on enabling and using Fleet Spaces in 8.16

[nimarezainia]

Fleet Space awareness is behind a feature flag in 8.16. Please find instructions below on how to enable this flag for use.

1. Go to Edit the deployment
2. Select the Kibana instance and Edit user settings:

3. To enable the feature flag, configure:
   xpack.fleet.enableExperimental: ['subfeaturePrivileges', 'useSpaceAwareness']

In the Security section of Stack Management, go to Roles and create a new role.
You will now have the ability to apply more granular privileges to the role in a space:

Once saved and confirmed your deployment will be ready.

4. Last step to enable the feature, please make the following API call:

curl -u elastic:your_deployment_password -XPOST "your_kibana URL/internal/fleet/enable_space_awareness" -H "kbn-xsrf: reporting" -H 'elastic-api-version: 1'

You should also now be able to configure the Agent Policy to be associated with multiple spaces. The agents enrolled in those policies will only be visible to users with access to the space the policy is in.

cc: @nchaulet

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)