Add KYAML support to kubectl

KYAML is a strict subset of YAML, which is sort of halfway between YAML
and JSON.  It has the following properties:
* Does not depend on whitespace (easier to text-patch and template).
* Always quotes value strings (no ambiguity aroud things like "no").
* Allows quoted keys, but does not require them, and only quotes them if
  they are not obviously safe (e.g. "no" would always be quoted).
* Always uses {} for structs and maps (no more obscure errors about
  mapping values).
* Always uses [] for lists (no more trying to figure out if a dash
  changes the meaning).
* When printing, it includes a header which makes it clear this is YAML
  and not ill-formed JSON.
* Allows trailing commas
* Allows comments,
* Tries to economize on vertical space by "cuddling" some kinds of
  brackets together.
* Retains comments.

Examples:

A struct:

```yaml
metadata: {
  creationTimestamp: "2024-12-11T00:10:11Z",
  labels: {
    app: "hostnames",
  },
  name: "hostnames",
  namespace: "default",
  resourceVersion: "15231643",
  uid: "f64dbcba-9c58-40b0-bbe7-70495efb5202",
}
```

A list of primitves:

```yaml
ipFamilies: [
  "IPv4",
  "IPv6",
]
```

A list of structs:

```yaml
ports: [{
  port: 80,
  protocol: "TCP",
  targetPort: 80,
}, {
  port: 443,
  protocol: "TCP",
  targetPort: 443,
}]
```

A multi-document stream:

```yaml
---
{
  foo: "bar",
}
---
{
  qux: "zrb",
}
```

Kubernetes-commit: 2cb955d8ccae30167b9610bfe51c2f86e83a1958

pkg/cmd/util/helpers.go

@@ -455,6 +455,12 @@ const (
 	// Transition to WebSockets.
 	RemoteCommandWebsockets FeatureGate = "KUBECTL_REMOTE_COMMAND_WEBSOCKETS"
 	PortForwardWebsockets   FeatureGate = "KUBECTL_PORT_FORWARD_WEBSOCKETS"
+
+	// owner: @thockin
+	// kep: https://kep.k8s.io/5296
+	//
+	// Support KYAML output.
+	KYAMLOutput FeatureGate = "KUBECTL_KYAML"
 )
 
 // IsEnabled returns true iff environment variable is set to true.