Improve quality of life for IncusOS users

[stgraber]

When running on top of an IncusOS host, there are a few things that we can tell requires additional configuration or that shouldn't be allowed in order to avoid breaking the system:

• Using the truenas storage driver should fail if the iscsi service isn't enabled
• Using the ceph, cephfs or cephobject storage drivers should fail if the ceph service isn't enabled
• Using the lvm or lvmcluster drivers should fail if the lvm service isn't enabled
• Using OVN networks should fail if the ovn service isn't enabled
• Using the ovs bridge type should fail if the ovn service isn't enabled
• Clearing the core.https_address key should not be allowed
• Removing the last client type certificate should not be allowed

As part of this, we should introduce an IncusOS client struct with convenience functions to check for a given service being enabled as well as to cover our current use case of filtering the network interface list.

None of this will have any impact to non-IncusOS users, but it will result in a much better user experience for those using it.