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
"index",
                        "delete",
                        IndicesAliasesAction.NAME,
                        UpdateSettingsAction.NAME
                    )
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices("logs-ti_*.*-*")
                    .privileges(
                        
                        DeleteIndexAction.NAME,
                        
                        "read",
                        "view_index_metadata"
                    )
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices("kibana_sample_data_*")
                    .privileges(
                        "create_index",
                        "delete_index",
                        "read",
                        "index",
                        "view_index_metadata",
                        IndicesAliasesAction.NAME,
                        UpdateSettingsAction.NAME
                    )
                    .build(),