connectivity: allow to restrict connectivity test pods using nodeSelector

[tklauser]

The connectivity test pods will be restricted to nodes matching labels given in the --node-selector flag.

Usage:

$ kubectl get nodes
NAME                 STATUS   ROLES           AGE   VERSION
kind-control-plane   Ready    control-plane   15m   v1.25.3
kind-worker          Ready    <none>          15m   v1.25.3
kind-worker2         Ready    <none>          15m   v1.25.3
kind-worker3         Ready    <none>          15m   v1.25.3
kind-worker4         Ready    <none>          15m   v1.25.3
$ kubectl label nodes kind-worker{2,3} connectivity.cilium.io/test=true
$ cilium connectivity test --node-selector connectivity.cilium.io/test=true
[...]
✅ All 32 tests (261 actions) successful, 2 tests skipped, 1 scenarios skipped.
$ kubectl get pods -n cilium-test -o wide
NAME                              READY   STATUS    RESTARTS   AGE     IP             NODE           NOMINATED NODE   READINESS GATES
client-7858556799-xd52x           1/1     Running   0          5m56s   10.244.2.120   kind-worker3   <none>           <none>
client2-646989676f-zgxxl          1/1     Running   0          5m56s   10.244.2.55    kind-worker3   <none>           <none>
echo-other-node-79659b5c6-8dzfw   2/2     Running   0          5m55s   10.244.3.148   kind-worker2   <none>           <none>
echo-same-node-69f7699d84-5bvxj   2/2     Running   0          5m56s   10.244.2.168   kind-worker3   <none>           <none>

Fixes #1269

[aanm]

@squeed FYI