feat(cdk/drag-drop): Allow dynamic drop lists creation while dragging an item

### Feature Description

I have noticed that if I try to create a drop list dynamically while I am dragging an item, the drop list is created graphically but it is not possible to drop the item I am dragging into that drop list. This seems to happen because the "_siblings" are created at drag initiation and do not notice if a row is created at runtime. It start working after a few drop list exit or enter but not if you directly go to that drop list.

Would it be possible to have this feature?
Or if it is already possible could you tell me how to do it?
For the moment I solved it by adding this method to my code, but it seems to me a very dirty solution.

```typescript
private refreshDropLists() {
// do it only if the component rows and the library rows are not sync
if (this.dropLists.toArray().length === this.configuration.length) {
// loop over the droplists
this.dropLists.toArray().forEach((dropList) => {
// force to recalculate the row positions
// @ts-ignore
dropList._dropListRef._cacheParentPositions();
// force the link between each row except for the row itself
dropList._dropListRef.connectedTo(
this.dropLists
.toArray()
.filter((dropList2) => dropList.id !== dropList2.id)
.map((droplist2) => droplist2._dropListRef)
);
});
}
}
```

An example can be viewed at the following link: https://stackblitz.com/edit/stackblitz-starters-7xvrf3?file=src%2Fapp%2Fapp.component.ts

If you want to make it work where to uncomment line 30 and 31 of the file "app.component.ts"

### Use Case

A useful use case would be, as with the linked stackblitz, creating a new droplists from the element I am dragging.
So if I have as in my case a structure of rows and columns, I want to be able to drag my column to a new row that doesn't exist.

Inizia con l'esempio StackBlitz collegato, in particolare src/app/app.component.ts, e riproduci la creazione di una drop list durante il trascinamento. Leggi la gestione di CDK drag-drop per gli elenchi fratelli e verifica che un elenco appena creato accetti un drop diretto senza il workaround; aggiungi o aggiorna test mirati se viene identificato il relativo punto di ingresso dei test.

Scritto dal modello di indicizzazione a partire dal testo della issue.