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Implements the server side changes for cilium/hubble#776

hubble: Add Support for filtering on HTTP headers

How to test this manually

1. Deploy cilium from source with make kind-image && make kind-install-cilium (note the hubble-export-file-path should be set for some weird reason that is not super clear to me, this is how the code flow works at

   cilium/daemon/cmd/hubble.go

   Line 173 in f078737

   if option.Config.HubbleExportFilePath != "" {

   )
2. Enable hubble with cilium hubble enable --chart-directory=/home/chrismark/go/src/github.com/cilium/cilium/install/kubernetes/cilium
3. Apply a L7 example endpoint with kubectl apply -f https://raw.githubusercontent.com/cilium/cilium/1.13.3/examples/minikube/sw_l3_l4_l7_policy.yaml
4. Port forward hubble relay svc with kubectl port-forward -n kube-system svc/hubble-relay --address 0.0.0.0 --address :: 4245:80
5. Deploy the sample app with kubectl create -f https://raw.githubusercontent.com/cilium/cilium/1.13.3/examples/minikube/http-sw-app.yaml. From https://docs.cilium.io/en/stable/gettingstarted/demo/#starwars-demo

-> Build hubble CLI from cilium/hubble#1277 after replacing the cilium dependency in the go.mod file with replace github.com/cilium/cilium => /home/chrismark/go/src/github.com/cilium/cilium.

7. Observe the flows with ./hubble observe --pod tiefighter --http-header Foo:bar -f
8. Execute a matching curl request with kubectl exec tiefighter -- curl -H "Foo: bar" -s -XPOST "http://deathstar.default.svc.cluster.local/v1/request-landing"
9. Verify that the flow is observed:

Oct 29 19:46:06.337: default/tiefighter:39348 (ID:49781) -> default/deathstar-7848d6c4d5-9z2wr:80 (ID:43688) http-request FORWARDED (HTTP/1.1 POST http://deathstar.default.svc.cluster.local/v1/request-landing)

10. Execute a non-matching curl request with kubectl exec tiefighter -- curl -H "Foo: baz" -s -XPOST "http://deathstar.default.svc.cluster.local/v1/request-landing"
11. Verify that the flow is not observed.