Closes: https://bugs.launchpad.net/netplan/+bug/1956264



COMMITS:
* Add tristate type for offload options
* Clarify name matching issues for `networkd`
* Fix tests for offload
* cli: apply: properly change udev/.link offloading settings
* Re-used existing offloading fields, they are ABI compatible
Size:
Both enum and gboolean reduce to integer, so they are of same size.
Content:
An old consumer looking at these will interpret UNSET as if it was TRUE,
which is the kernel's default (=UNSET) value.
* CI: quirk to add ethtool test dependency
* tests: ethernets: link offloading validation
* doc: be more specific with the offloading docs

Co-authored-by: Lukas Märdian <slyon@ubuntu.com>