tailcfg,cmd/k8s-operator,kube: move Kubernetes cap to a location that can be shared with control (#12236)

This PR is in prep of adding logic to control to be able to parse
tailscale.com/cap/kubernetes grants in control:
- moves the type definition of PeerCapabilityKubernetes cap to a location
shared with control.
- update the Kubernetes cap rule definition with fields for granting
kubectl exec session recording capabilities.
- adds a convenience function to produce tailcfg.RawMessage from an
arbitrary cap rule and a test for it.

An example grant defined via ACLs:
"grants": [{
      "src": ["tag:eng"],
      "dst": ["tag:k8s-operator"],
      "app": {
        "tailscale.com/cap/kubernetes": [{
            "recorder": ["tag:my-recorder"]
	    “enforceRecorder”: true
        }],
      },
    }
]
This grant enforces `kubectl exec` sessions from tailnet clients,
matching `tag:eng` via API server proxy matching `tag:k8s-operator`
to be recorded and recording to be sent to a tsrecorder instance,
matching `tag:my-recorder`.

The type needs to be shared with control because we want
control to parse this cap and resolve tags to peer IPs.

Updates tailscale/corp#19821

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

cmd/k8s-operator/proxy.go

@@ -20,6 +20,7 @@ import (
 	"k8s.io/client-go/transport"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/client/tailscale/apitype"
+	tskube "tailscale.com/kube"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tsnet"
 	"tailscale.com/util/clientmetric"
@@ -211,31 +212,20 @@ const (
 	// tailfcg.PeerCapabilityKubernetes capability. The only capability rule
 	// that is respected for this form is group impersonation - for
 	// backwards compatibility reasons.
+	// TODO (irbekrm): determine if anyone uses this and remove if possible.
 	oldCapabilityName = "https://" + tailcfg.PeerCapabilityKubernetes
 )
 
-type capRule struct {
-	// Impersonate is a list of rules that specify how to impersonate the caller
-	// when proxying to the Kubernetes API.
-	Impersonate *impersonateRule `json:"impersonate,omitempty"`
-}
-
-// TODO(maisem): move this to some well-known location so that it can be shared
-// with control.
-type impersonateRule struct {
-	Groups []string `json:"groups,omitempty"`
-}
-
 // addImpersonationHeaders adds the appropriate headers to r to impersonate the
 // caller when proxying to the Kubernetes API. It uses the WhoIsResponse stashed
 // in the context by the apiserverProxy.
 func addImpersonationHeaders(r *http.Request, log *zap.SugaredLogger) error {
 	log = log.With("remote", r.RemoteAddr)
 	who := whoIsKey.Value(r.Context())
-	rules, err := tailcfg.UnmarshalCapJSON[capRule](who.CapMap, tailcfg.PeerCapabilityKubernetes)
+	rules, err := tailcfg.UnmarshalCapJSON[tskube.KubernetesCapRule](who.CapMap, tailcfg.PeerCapabilityKubernetes)
 	if len(rules) == 0 && err == nil {
 		// Try the old capability name for backwards compatibility.
-		rules, err = tailcfg.UnmarshalCapJSON[capRule](who.CapMap, oldCapabilityName)
+		rules, err = tailcfg.UnmarshalCapJSON[tskube.KubernetesCapRule](who.CapMap, oldCapabilityName)
 	}
 	if err != nil {
 		return fmt.Errorf("failed to unmarshal capability: %v", err)