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
"synthetics-icmp-*",
                        "synthetics-tcp-*",
                        "synthetics-browser-*",
                        "synthetics-browser.network-*",
                        "synthetics-browser.screenshot-*"
                    )
                    .privileges(DeleteIndexAction.NAME)
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices("metrics-endpoint.metadata*")
                    .privileges("read", "view_index_metadata")
                    .build(),
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(
                        "metrics-endpoint.metadata_current_default*",
                        ".metrics-endpoint.metadata_current_default*",
                        ".metrics-endpoint.metadata_united_default*"
                    )
                    .privileges("create_index", "delete_index", "read", "index", IndicesAliasesAction.NAME, UpdateSettingsAction.NAME)
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices("logs-ti_*_latest.*")
                    .privileges(
                        
                        
                        "create_index",
                        "delete_index",
                        "read",