Integrate k3s and multus

[caroline-suse-rancher]

An issue to track the work being done in #9434

To do list:

• ADR about multus integration in K3s
• Multus integration code
• Multus e2e test
• Change the release scripts to generate a new tarball with the multus images for airgap (it will require a docs update)

[caroline-suse-rancher]

Putting this on hold until @manuelbuil is back, or someone else takes over

[manuelbuil]

I think we decided not to proceed with this because it was rather complicated

[brandond]

Yeah I think the biggest problem is just how we don't have a consistent path for our CNI bins, which has been an issue for community members as well. The bin dir changes every time K3s is upgraded.

We might consider using a stable bin dir for CNIs, and just updating the symlinks or overwriting the binaries when k3s is upgraded.