kubernetes charms don't set ipvs mode (code and docs bug)

Hi,

from https://charmhub.io/kubernetes-control-plane IPVS can be enabled by

juju config kubernetes-control-plane proxy-extra-args="proxy-mode=ipvs"

unfortunately this is not working because if you have a look at the kube-proxy man page https://kubernetes.io/docs/reference/command-line-tools-reference/kube-proxy/

--proxy-mode ProxyMode
Which proxy mode to use: on Linux this can be 'iptables' (default) or 'ipvs'. On Windows the only supported
value is 'kernelspace'.This parameter is ignored if a config file is specified by --config.

the unit uses this command line:

/snap/kube-proxy/3038/kube-proxy --v=0 --master=https://10.0.0.143:6443 --config=/root/cdk/kubeproxy/config.yaml --proxy-mode=ipvs

so proxy-mode extra argument is ignored.

how to fix:
1) there are 2 missing packages: ipset e conntrack (the latter is a very old standing issue, for instance https://github.com/kubernetes/kubernetes/pull/26839/files)
2) the /root/cdk/kubeproxy/config.yaml should be completed with:
mode: "ipvs"
ipvs:
  strictARP: true

instead of dealing with extra args, as we need to install (and remove) packages, it may be good to have a dedicated flag such as "proxy-mode" with ["iptables","ipvs"] options.

when switching from iptables to ipvs you need to flush iptables/ipvs rules (while testing i fixed with a basic machine reboot).

the kubernetes-worker charm shares exactly the same issue.