[List] support "expand and collapse" from List spec

#### Please describe the feature you would like to request.
The [List spec](https://material.io/design/components/lists.html#types) includes an "Expand and collapse" control. This functions more or less like an Expansion Panel but with List Item styling: dividers are used instead of a surrounding border / different background color, and the expand/collapse button is placed according to the rules for secondary action buttons.

#### What is the use-case or motivation for this proposal?
Support full capabilities of List as described in the spec

#### Is there anything else we should know?
I put together [a StackBlitz](https://stackblitz.com/edit/angular-material2-issue-list-expand) showing the current behavior when an expansion panel is placed in a list. The behavior is more or less as expected but the styling does not work as a list item, and just "fixing up" the current behavior is a lot to ask of consumers.

In my own project I built a component to implement this behavior, but it's not going to be as robust as it would be if supported directly here. Of course, styling an expansion panel to behave correctly when used as a list item is only one approach, and I don't mean to suggest a specific design. My custom component tracks its own expansion state and handles divider logic, etc, internally.

Comienza con el StackBlitz enlazado para reproducir el comportamiento actual y, a continuación, compara los componentes List y Expansion Panel existentes con la especificación de List enlazada. El trabajo estará terminado cuando el comportamiento de expandir/contraer, los separadores, el estilo de los elementos de la lista y la ubicación del botón de acción secundaria coincidan con el diseño solicitado; el payload no especifica archivos ni pruebas concretos.

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