In order to debug Relay to Hubble connectivity issues, it is sometimes
useful to have a Pod running with the Hubble CLI.

Because the Relay image is based on a scratch image, kubectl exec'ing
into it is not possible. While the Hubble CLI can be found in the Cilium
Pods, the Relay certificate needed to establish the mTLS handshake to
the Hubble server is not mounted into the Cilium Pods.

This commit introduce a new hubble-cli.yaml file. When debugging Relay
mTLS issues, it can be used to quickly run a hubble-cli Pod:

    kubectl create -f /path/to/hubble-cli.yaml

Since the Relay mTLS certificates are mounted into the hubble-cli Pod,
one can connect to a Hubble server given it's IP address:

    kubectl exec -it -n kube-system hubble-cli -- \
        hubble observe --server tls://${IP?}:4244 \
            --tls-server-name worker0.default.hubble-grpc.cilium.io \
            --tls-ca-cert-files /var/lib/hubble-relay/tls/hubble-server-ca.crt \
            --tls-client-cert-file /var/lib/hubble-relay/tls/client.crt \
            --tls-client-key-file /var/lib/hubble-relay/tls/client.key

Signed-off-by: Alexandre Perrin <alex@kaworu.ch>