As part of the `unified limits` keystone has added a `registered_limits` api. Add support for it. [docs](https://docs.openstack.org/api-ref/identity/v3/?expanded=list-projects-detail#unified-limits) [code](https://github.com/openstack/keystone/blob/34a37772e866074f0bc5413c18bb446ac8c1d17e/keystone/api/registered_limits.py)