IP configuration method.
SettingIP4Config and SettingIP6Config both support "disabled", "auto", "manual", and "link-local". See the subclass-specific documentation for other values.
In general, for the "auto" method, properties such as dns and routes specify information that is added on to the information returned from automatic configuration. The ignore_auto_routes and ignore_auto_dns properties modify this behavior.
For methods that imply no upstream network, such as "shared" or "link-local", these properties must be empty.
For IPv4 method "shared", the IP subnet can be configured by adding one manual IPv4 address or otherwise 10.42.x.0/24 is chosen. Note that the shared method must be configured on the interface which shares the internet to a subnet, not on the uplink which is shared.