feat(CdkObserveContent): Enable observing Resize using ResizeObserver

#### 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.

Inizia leggendo le direttive e i servizi CdkObserveContent attuali, quindi confronta la loro struttura con il caso d’uso di ResizeObserver descritto. Determina quali parti devono diventare generiche per supportare altre Observer APIs e verifica che il comportamento esistente di osservazione del contenuto rimanga invariato.

Scritto dal modello di indicizzazione a partire dal testo della issue.