binding classes for component activated by router-outlet

### Which @angular/* package(s) are relevant/related to the feature request?

router

### Description

For an Angular UI library like [Clarity](https://clarity.design/), regarding the [application layout](https://clarity.design/documentation/app-layout), `.content-container` class is required for the top level route components.

I'd like to propose a feature request about _binding classes for component activated by `router-outlet`_.

### Proposed solution

Add an `@Input() componentClass = ''` to `router-outlet` directive.

Aafter `this.activated` is initialized, add the classes to the created or attached component as following:

```typescript
this.activated.location.nativeElement.classList.add(...this.componentClass.split(' '));
```

### Alternatives considered

Currently, as mentioned above, for each route component, the following codes should be added in the `@Component` metadata:

```
host: {
'[class.content-container]': 'true',
},
```

Adding `componentClass` to router-outlet solves it more directly and concisely.

Start at the router-outlet directive entry point and inspect how activated or attached components are handled. Determine where a componentClass input would apply classes and how existing router-outlet tests cover activation; done means the requested classes are applied to routed components without breaking existing behavior.

Written by the indexing model from the issue text.