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.

まず現在の CdkObserveContent ディレクティブとサービスを読み、その構造を、説明されている ResizeObserver のユースケースと比較してください。他の Observer APIs をサポートできるようにするために、どの部分を汎用化する必要があるかを判断し、既存のコンテンツ監視の動作が維持されることを確認してください。

索引モデルが issue の本文から書いたものです。