feat(cdk-menu): enable library devs to provide default items

[spike-rabbit]

Feature Description

As a library developer I want to build a menu around the cdkMenu which has some default items.

It is not possible to achieve it like this:

my-lib.component.html

<div cdkMenu>
  <ng-content></ng-content><!--  dynamic items from library consumer -->
  <div cdkMenuItem>default</div>
</div>

There is a runtime error saying the menu-stack cannot be injected. I guess the reason for this is: angular/angular#8563

Another approach would is to attach the ckdMenu as host directive to my-lib component. Then the ContentChildren are resolved, but the default item is not, because it is a view child.

I don't care how, but it would be cool if it is somehow is possible to have default items.

Use Case

I am part of a team that maintains an opinionated component lib for internal usage. We would like to replace most parts of our own menu implementation with the cdkMenu. We have some default items that every consuming application must have.