cmd/k8s-operator,k8s-operator/sessionrecording: support recording kubectl exec sessions over WebSockets (#12947)

cmd/k8s-operator,k8s-operator/sessionrecording: support recording WebSocket sessions

Kubernetes currently supports two streaming protocols, SPDY and WebSockets.
WebSockets are replacing SPDY, see
https://github.com/kubernetes/enhancements/issues/4006.
We were currently only supporting SPDY, erroring out if session
was not SPDY and relying on the kube's built-in SPDY fallback.

This PR:

- adds support for parsing contents of 'kubectl exec' sessions streamed
over WebSockets

- adds logic to distinguish 'kubectl exec' requests for a SPDY/WebSockets
sessions and call the relevant handler

Updates tailscale/corp#19821

Signed-off-by: Irbe Krumina <irbe@tailscale.com>
Co-authored-by: Tom Proctor <tomhjp@users.noreply.github.com>

k8s-operator/sessionrecording/fakes/fakes.go

@@ -13,6 +13,9 @@ import (
 	"net"
 	"sync"
 	"testing"
+	"time"
+
+	"math/rand"
 
 	"tailscale.com/sessionrecording"
 	"tailscale.com/tstime"
@@ -116,3 +119,20 @@ func AsciinemaResizeMsg(t *testing.T, width, height int) []byte {
 	}
 	return append(bs, '\n')
 }
+
+func RandomBytes(t *testing.T) [][]byte {
+	t.Helper()
+	r := rand.New(rand.NewSource(time.Now().UnixNano()))
+	n := r.Intn(4096)
+	b := make([]byte, n)
+	t.Logf("RandomBytes: generating byte slice of length %d", n)
+	_, err := r.Read(b)
+	if err != nil {
+		t.Fatalf("error generating random byte slice: %v", err)
+	}
+	if len(b) < 2 {
+		return [][]byte{b}
+	}
+	split := r.Intn(len(b) - 1)
+	return [][]byte{b[:split], b[split:]}
+}