Mat Stepper needs a beforeSelectionChange event

### Feature Description

There should be an event that is triggered when you attempt to navigate to a different step via the mat stepper header.

Currently, there is only a `selectionChange` event that doesn't get triggered until _after_ the next navigation takes place.

### Use Case

In my scenario, when a user attempts to navigate to the next step, I need to first call an endpoint to validate the data they entered. Once I have validated that the data is correct, I can proceed to the next step with `stepper.next()`. This works as expected if the user clicks on my next button with a custom `(click)` event, but if they click on the header to navigate, there is no way to call the endpoint before navigation.

Using `selectionChange` does not work, because there is no way to prevent navigation after the `StepperSelectionEvent` has happened.

Alternatively, instead of a `beforeSelectionChange` event, if there were a way to prevent the next navigation when subscribing to the `selectionChange` event, that would also work. Or if there was a way to prevent navigation from the header, but still capture clicks, so that `stepper.next` could manually be called.

Beginne mit der Header-Navigation des mat stepper und dem bestehenden selectionChange-Ereignis, und vergleiche dann diesen Ablauf mit dem im Issue beschriebenen Einstiegspunkt stepper.next(). Die Änderung ist abgeschlossen, wenn die Header-Navigation vor der Auswahl validiert oder verhindert werden kann, während die programmgesteuerte Navigation zum nächsten Schritt weiterhin funktioniert.

Vom Indexierungsmodell aus dem Issue-Text verfasst.