feat(CdkPortalOutlet): Introduce signal-friendly way to track portal attachment/detachment

### Feature Description

Tracking if a portal is attached/detached within a `CdkPortalOutlet` can be cumbersome, it would be a nice improvement to expose the attached portal or the attachment/detachment state as a signal.

### Use Case

We create a component portal on a parent element which will get components attached by its children. In case a component is attached, I need to adapt the styling.

Lets take the following example, my expectation initially was that the class will be set correctly.

```
myOutlet = viewChild(CdkPortalOutlet);

// outlet gets passed to children which will attach components

```
The issue though is that `myOutlet` is only initialized once and changes to the `portal` within are not tracked if there is no other event triggering change detection at the same time. So the code above can actually get out-of-sync.

The best workaround I found is to manually keep track of the attachment/detachment.
This is more complex than it should be and my example is also possibly missing cleanup code

```
myOutlet = viewChild(CdkPortalOutlet);
isOutletAttached = signal(false);

effect(() => {
this.myOutlet()?.attached.subscribe(() => {
this.isOutletAttached.set(true);
this.myOutlet().attachedRef?.onDestroy(() => this.isOutletAttached.set(false));
});
});

...
```

Ideally one of the internal members of the `CdkPortalOutlet` should expose a signal which can be used.

```
myOutlet = viewChild(CdkPortalOutlet);

// multiple options
...
...
...
```

Comienza con el punto de entrada CdkPortalOutlet mencionado en el issue y sigue cómo se expone el estado de adjunción y desadjunción de su portal. Compara las formas de signal propuestas con el comportamiento existente de attached y attachedRef. Se considera terminado cuando los consumidores pueden reaccionar a los cambios de adjunción sin suscripciones ni limpieza manuales, y la API elegida y su comportamiento están documentados por las pruebas del proyecto.

Escrito por el modelo de indexación a partir del texto del issue.