feat(Platform): allow DI override of userAgent

### Feature Description

Right now it is hardcoded as global `navigator.userAgent`:
https://github.com/angular/components/blob/main/src/cdk/platform/platform.ts

However in SSR there's a way to get userAgent from the request and accessing global entities is bad practice. Would be great if there was a token for userAgent [like we do here](https://github.com/taiga-family/ng-web-apis/blob/main/libs/common/src/tokens/user-agent.ts), or maybe at least `inject(DOCUMENT).defaultView?.navigator?.userAgent` way of accessing it, rather than just global `navigator.userAgent`

### Use Case

_No response_

Start in src/cdk/platform/platform.ts, where the global navigator.userAgent access is defined, and inspect the surrounding platform injection setup. Compare the requested injectable user-agent source with the existing DOCUMENT access and verify the result in SSR while preserving browser behavior; done means callers can override the user agent through DI.

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