Add command to show CiliumIdentity statistics

[yokaze]

This PR adds the following two commands:

• npv id summary: count CiliumIdentity by namespace

$ npv id summary
NAMESPACE          IDENTITY
default            1
kube-system        2
local-path-storage 1
test               12

• npv id label: show cardinality and values of security-relevant labels in a namespace

$ npv id label -n test
LABEL                                   COUNT VALUES
k8s:group                               1     test
k8s:io.cilium.k8s.policy.serviceaccount 1     default
k8s:test                                12    l3-egress-explicit-deny-all,l3-egress-implicit-deny-all,l3-ingress-explicit-allow-all,l3-ingress-explicit-deny-all,l3-ingress-implicit-deny-all,l4-egress-explicit-deny-any,l4-egress-explicit-deny-tcp,l4-ingress-explicit-allow-any,l4-ingress-explicit-allow-tcp,l4-ingress-explicit-deny-any,l4-ingress-explicit-deny-udp,self

[chez-shanpu]

LGTM