Add to the Server type the list of server groups the server is part of,
as exposed in microversions 1.71 and above as "server_groups".

The new `ServerGroups` property is a pointer to slice (`*[]string`), as
mandated by the Gophercloud ["New Response Fields" convention][1].

Nova code implementing the feature:
* https://github.com/openstack/nova/blob/d2a5fe5621d6ff1ae8ba5087049e0c4347592cf6/nova/api/openstack/compute/servers.py#L454-L455
* https://github.com/openstack/nova/blob/52cae8801de9229d6cfb7871c2073b83b3e41b81/nova/api/openstack/compute/views/servers.py#L420-L423

[1]: https://github.com/gophercloud/gophercloud/blob/master/docs/MICROVERSIONS.md#new-response-fields

Now we should be able to do something like:

```
updateOpts := images.UpdateOpts{
    images.ReplaceImageProtected{
        NewProtected: false,
    },
}
image, err = images.Update(client, imageID, updateOpts).Extract()
```

Fixes #2220

This change exposes the error message of NotFound error from openstack
API, this is helpful to find out the reason of request failure.

This commit adds support for HttpHeaders in the create_subscription.
It was added during the Xena release of Ironic in [1] after the
initial support for create subscriptions [2]

We don't need to modify the response of get_subscription because
redfish doesn't return the value of the HttpHeaders for the enduser.

[1] https://review.opendev.org/c/openstack/ironic/+/806859
[2] https://review.opendev.org/c/openstack/ironic/+/801064