Add STACKIT

[robinschneider]

Add new provider STACKIT for testing.

Add CI automation for vendor testing of provider STACKIT.

How to use

Run in CI or locally with all required environment variables.

Testing done

[Describe the testing you have done before submitting this PR. Please include both the commands you issued as well as the output you got.]

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

[tormath1]

Thanks, it looks good. We just need to plug the garbage collection as well (you can see here: https://github.com/flatcar/scripts/pull/2873/files how it's done)

[tormath1]

So amd64 testing is working fine:

1..31
ok - cl.ignition.v2.noop
ok - cl.internet
ok - docker.network-openbsd-nc
ok - cl.ignition.kargs
ok - cl.basic
ok - coreos.ignition.resource.remote
ok - cl.ignition.misc.empty
ok - coreos.ignition.sethostname
ok - linux.nfs.v3
ok - cl.ignition.v2.btrfsroot
ok - cl.flannel.vxlan
ok - cl.ignition.v1.noop
ok - cl.install.cloudinit
ok - cl.flannel.udp
ok - kubeadm.v1.32.4.cilium.base
ok - kubeadm.v1.33.0.flannel.base
ok - cl.cloudinit.basic
ok - kubeadm.v1.33.0.cilium.base
ok - linux.nfs.v4
ok - kubeadm.v1.33.0.calico.base
ok - coreos.ignition.ssh.key
ok - coreos.ignition.resource.local
ok - coreos.ignition.once
ok - kubeadm.v1.32.4.flannel.base
ok - kubeadm.v1.34.1.cilium.base
ok - cl.network.initramfs.second-boot
ok - cl.etcd-member.discovery
ok - coreos.ignition.security.tls
ok - kubeadm.v1.34.1.flannel.base
ok - kubeadm.v1.32.4.calico.base
ok - kubeadm.v1.34.1.calico.base

If we want to enable ARM64 testing, we need to proceed similar to Hetzner. First we add the two instances types in the following format:

scripts/ci-automation/ci-config.env

Lines 150 to 151 in 8453fd7

	: ${HETZNER_amd64_INSTANCE_TYPE:="cpx11"}
	: ${HETZNER_arm64_INSTANCE_TYPE:="cax11"}

Then we set the instance type based on the arch:

scripts/ci-automation/vendor-testing/hetzner.sh

Lines 13 to 14 in 8453fd7

	hetzner_instance_type_var="HETZNER_${CIA_ARCH}_INSTANCE_TYPE"
	hetzner_instance_type="${!hetzner_instance_type_var}"

To finally pass it to Mantle:

scripts/ci-automation/vendor-testing/hetzner.sh

Line 56 in 8453fd7

--hetzner-server-type="${hetzner_instance_type}" \