Allow content to be projected as list header and list footer in material dropdown select. Different from header and footer, these two sections scroll with the list.
Allow content to be projected as list header and list footer in material dropdown select.
Different from header and footer, these two sections scroll with the list.

The list header can be used to support custom deselect component.
The list footer can be used to support infinite load.

PiperOrigin-RevId: 187582134

angulardart/angular_components,angulardart/angular_components,angulardart/angular_components