Support providing multiple drag placeholders

#### Please describe the feature you would like to request.
Currently there is one type of place holder, the one that's defined with the `*cdkDragPlaceholder` directive.
However the destination container might display the `cdkDrag` item different than the original container.

You can see the issue in here:
https://stackblitz.com/edit/angular-terlwz

If you'll try to drag to the done list the place holder there doesn't show the right layout of the component if it would be dropped.

#### What is the use-case or motivation for this proposal?
I need to take item from one container with a certain layout, and drop the element in other container with different layout. since the layout is different and the way the drag item is viewed in the container is different I suppose the place holder also need to be different.

#### Is there anything else we should know?
I'm also using the drag preview for other feature so I can't use it here.
Maybe the container itself should accept a template for rendering an element that is about to be added.

What do you think?

首先复现所链接 StackBlitz 示例中的行为，并阅读拖动占位符指令和目标容器的行为。当被拖动的项目能够使用适合其进入的容器的占位符布局，而不依赖现有的拖动预览时，即表示完成。

由索引模型根据 Issue 内容生成。