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

从 issue 中提到的入口点 MatIconRegistry.addSvgIconSet 和 getNamedSvgIcon 开始，跟踪现有的图标集加载工作流。完成的内容应包括用于加载一个或多个图标集文件的明确反应式完成信号，并且应对该行为进行文档说明，或由 registry 的现有测试覆盖。

由索引模型根据 Issue 内容生成。