Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStore.java
"monitor",
                "manage_index_templates",
                MonitoringBulkAction.NAME,
                "manage_saml",
                "manage_token",
                "manage_oidc",
                
                "manage_pipeline",
                "manage_ilm",
                
                "manage_transform",
                InvalidateApiKeyAction.NAME,
                "grant_api_key",
                "manage_own_api_key",
                GetBuiltinPrivilegesAction.NAME,
                "delegate_pki",
                GetProfilesAction.NAME,
                ActivateProfileAction.NAME,
                SuggestProfilesAction.NAME,
                ProfileHasPrivilegesAction.NAME,
                
                "manage_ml",
                
                "cluster:admin/analyze",
                
                "monitor_text_structure",
                
                "cancel_task" },
            new RoleDescriptor.IndicesPrivileges[] {