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

Start with the MatIconRegistry.addSvgIconSet and getNamedSvgIcon entry points named in the issue, and trace the existing icon-set loading workflow. Done should include a clear reactive completion signal for loading one or more icon-set files, with behavior documented or covered by the registry's existing tests.

Written by the indexing model from the issue text.