cmd/k8s-operator,k8s-operator/sessionrecording,sessionrecording,ssh/tailssh: refactor session recording functionality (#12945)

cmd/k8s-operator,k8s-operator/sessionrecording,sessionrecording,ssh/tailssh: refactor session recording functionality

Refactor SSH session recording functionality (mostly the bits related to
Kubernetes API server proxy 'kubectl exec' session recording):

- move the session recording bits used by both Tailscale SSH
and the Kubernetes API server proxy into a shared sessionrecording package,
to avoid having the operator to import ssh/tailssh

- move the Kubernetes API server proxy session recording functionality
into a k8s-operator/sessionrecording package, add some abstractions
in preparation for adding support for a second streaming protocol (WebSockets)

Updates tailscale/corp#19821

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

ssh/tailssh/tailssh_test.go

@@ -36,6 +36,7 @@ import (
 	"tailscale.com/ipn/store/mem"
 	"tailscale.com/net/memnet"
 	"tailscale.com/net/tsdial"
+	"tailscale.com/sessionrecording"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tempfork/gliderlabs/ssh"
 	"tailscale.com/tsd"
@@ -630,7 +631,7 @@ func TestSSHRecordingNonInteractive(t *testing.T) {
 	wg.Wait()
 
 	<-ctx.Done() // wait for recording to finish
-	var ch CastHeader
+	var ch sessionrecording.CastHeader
 	if err := json.NewDecoder(bytes.NewReader(recording)).Decode(&ch); err != nil {
 		t.Fatal(err)
 	}