feat(CdkObserveContent): Enable observing Resize using ResizeObserver

[twjacobsen]

Feature Description

It would be great, if the current implementation of CdkObserveContent could be made more generic, so that it could be used for implementing other *Observer APIs.

Use Case

On my project, we need to observe the resizing of an element - ResizeObserver is great for this. But it's cumbersome to work with on it's own.

I've added some directives and services that basically just extend and wrap the current CdkObserveContent services and directives, which seems to work, but there's some typecasting and I/O handling that is not very pretty. It looks like it would be a fairly small effort to make the generic parts of the current implementation properly generic, which would make it a breeze to support other observer APIs.

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[MikaStark]

I think it would be great to have public access to ResizeObserver features from this file : src/cdk/observers/private/shared-resize-observer.ts.