This PR updates the Pro support to use the 'auto-attach' feature [0] that is recently available. The advantage of using this functionality is that we don't need the contract id, which means that we don't need root to enable pro on the instance. The only requirement is that guest attachment has been enabled on the host before running. Note that this is targeting the `feature/pro-sources`, which is very much out of date (still using `tox`, for example). Since the feature-branch code is working, I thought it would be better to do this change here (where we can verify that it works), and *then* have a task to update & merge back into `main`, rather than the other way around. As far as I can tell the failure is unrelated and will "probably" be fixed when we merge `main` back in. 0: https://documentation.ubuntu.com/lxd/latest/howto/instances_ubuntu_pro_attach/