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

Comienza con los puntos de entrada MatIconRegistry.addSvgIconSet y getNamedSvgIcon mencionados en el issue, y sigue el flujo de trabajo existente para cargar conjuntos de iconos. La tarea debería incluir una señal reactiva clara de finalización para la carga de uno o más archivos de conjuntos de iconos, con el comportamiento documentado o cubierto por las pruebas existentes del registro.

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