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
.privileges(UpdateSettingsAction.NAME, PutMappingAction.NAME, RolloverAction.NAME)
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices(".logs-endpoint.action.responses-*").privileges("read").build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".logs-endpoint.actions-*")
                    .privileges("auto_configure", "read", "write")
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".logs-osquery_manager.action.responses-*")
                    .privileges("auto_configure", "create_index", "read", "index", "delete")
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".logs-osquery_manager.actions-*")
                    .privileges("auto_configure", "create_index", "read", "index", "write", "delete")
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(
                        ".logs-endpoint.diagnostic.collection-*",
                        "logs-apm-*",
                        "logs-apm.*-*",
                        "metrics-apm-*",
                        "metrics-apm.*-*",
                        "traces-apm-*",
                        "traces-apm.*-*",
                        "synthetics-http-*",