Port GCE OEM setup to systemd-sysext image

[pothos]

As was already done for the Azure OEM setup in #769 we want to port the GCE OEM setup over to systemd-sysext to benefit from updates.

A benefit of sysext images is that they can have a strict coupling to the Flatcar VERSION_ID which enables to use dynamical linking to save space and make the OEM tool packages simpler: we can remove the GCE nspawn "container".
The base Ignition configuration is also not needed anymore because the sysext's OEM files already become part of the /usr hierarchy and systemd services there can get started through an Upholds= drop-in.
For the /etc/profile.d/google-cloud-sdk.sh file we probably need a helper systemd unit that sets up symlinks to the file in the sysext image. The ssh service enablement is common for many OEMs and we should cover this from the base image as suggested in #1142.

The migration action has to be defined in the update_engine flatcar-postinst hook like it was done for Azure.