Add suppport for set expiration of image

[Cynerd]

With https://github.com/lxc/lxd/pull/6266 it is possible to modify the expiration date of the image and with https://github.com/lxc/lxd/pull/8831 it is possible to set an expiration date on publishing. This is not yet supported by pylxd (at least if I grepped right).

I think that the needs two changes:

• Image attribute expires_at should no longer be read-only
• The expires attribute should be added to publish method of the container and value propagated to API call.