[Drag&Drop] Use descendants: true when dealing with ContentChildren in most drap&drop directives

[brgrz]

Feature Description

The change to descendants: true for drag handle in Drag directive was welcoming change. Allow for deeper traversal using descendants:true for other content children also, for example cdkDragPreview and cdkDragPlaceholder.

Use Case

Advanced component nesting scenarios are currently blocked or require cumbersome workarounds like slot projections as pictured here #13360 (comment)

When most issues would be solved with deep traversal for content children.

Also, the new policy of locking issues which already cover the topic like here #13784 does not help the conversation at all.

[brgrz]

Just found out something similar was requested here #14939

[angular-automatic-lock-bot]

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