feat(MatIconRegistry): reactive signatures

### Feature Description

When I use MatIconRegistry, and I add an "iconSet" using "addSvgIconSet", I don't see any way of subscribing to what's internally doing: loading file -> generating an html/svg representation -> etc

And I don't know when all that computation has finished.

I'm using a little "hack" to subscribe to that workflow using:

`matIconRegistry.getNamedSvgIcon().subscribe(...)`

But that's kinda weak since I have to know the contents (what SVGs are included) in the iconSet.

I would like to have a robust API so that I can subscribe to that internal workflow and know when it has finished its calculations, something like:

`matIconRegistry.addSvgIconSet().subscribe(...)`

Even more interesting, allow this registry to load multiple files at once:

`matIconRegistry.addSvgIconSet([, ...]).subscribe(...)`

### Use Case

Synchronize APP Icons configuration at bootstrap.

Thanks

Commencez par les points d’entrée MatIconRegistry.addSvgIconSet et getNamedSvgIcon mentionnés dans l’issue, puis suivez le workflow existant de chargement des ensembles d’icônes. Le travail doit inclure un signal réactif clair indiquant la fin du chargement d’un ou plusieurs fichiers d’ensemble d’icônes, avec un comportement documenté ou couvert par les tests existants du registre.

Rédigé par le modèle d'indexation à partir du texte de l'issue.