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

Inizia dai punti di ingresso MatIconRegistry.addSvgIconSet e getNamedSvgIcon indicati nell’issue e traccia il workflow esistente per il caricamento dei set di icone. Il lavoro completato dovrebbe includere un segnale reattivo chiaro del completamento del caricamento di uno o più file di set di icone, con il comportamento documentato o coperto dai test esistenti del registry.

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