Rethink KPR sub-flags

[brb]

Currently, the kube-proxy-replacement feature exposes bunch of cilium-agent flags used to configure the feature. The amount of flags proven to be one of the main sources of confusion when using the feature.

In v1.19 we want to change the semantics of the --kube-proxy-replacement=boolean.

• true - enable handling of all service types (ClusterIP, NodePort, LoadBalancer) for E/W traffic (Pod) and N/S (external clients).
• false - only ClusterIP services are handled for Pods (we cannot completely remove the service handling because of Disable bpf_lxc loadbalancing (service translation) when KPR=false #16197).

Next, we want to unconditionally enable some of the features, for example session affinity, source range checks, internal-traffic-policy.

The changes will allow us to get rid of multiple flags, which were already deprecated for the upcoming v1.18, for example --enable-{host-port,node-port,externalips}. See the recent pull requests for more - https://github.com/cilium/cilium/pulls?q=is%3Apr+author%3Abrb+deprecate.

[pchaigno]

@brb What's the status of this given #41238? What remains to be done?

[brb]

The final boss - #39582 😄