* ABI compatibility checker (canonical#269)

* abigail: check-in 0.104 ABI

XML serialization generated on Focal via:
abidw libnetplan.so.0.0 --headers-dir include/ > abi-compat/focal_0.104.xml

* abi-compat: use abigail to check ABI compatibility via GA

* [WIP] src:types: move netplan_net_definition into a separate file

This is to allow ABI compatibilty checks of NetplanNetDefinition using the
'abidiff' tool.

* WIP: keep the ABI-specific structs self-contained

Also, only move them in include/ for the ABI check, so that they don't
get shipped as public headers.

* GA CI: update comments

* abi-compat: update XML according to latest state

* GA: upgrade to abigail-tools 2.0

Using PPA version from https://launchpad.net/~slyon/+archive/ubuntu/ci-netplan

* src: cleanup types.h ABI include

Co-authored-by: Simon Chopin <simon.chopin@canonical.com>

* networkd: rename GatewayOnlink= to GatewayOnLink= (canonical#273)

As of systemd v242 the "GatewayOnlink" spelling was deprecated and replaced by
"GatewayOnLink", while still being available in compat mode.

We've waited long enough and can switch to the proper spelling now.

systemd/systemd@9cb8c55

* cli:sriov: fix test coverage for quirk_devices

newer coverage tools seem to be more picky, make them happy

* Makefile: clean src/_features.h.gch file

* Adding VRF and VXLAN support for systemd-networkd. LP: #1764716, #1773522

* Add vxlan/vrf changes to new abi.h file

Co-authored-by: Lukas Märdian <slyon@ubuntu.com>
Co-authored-by: Simon Chopin <simon.chopin@canonical.com>
Co-authored-by: Anthony Timmins <atimmins@datto.com>