Add support for adding the spinner to the CDK overlay

[UncleRandom]

While using the spinner as a full page spinner it would be great if the spinner could be added as part of the CDK overlay rather than within the specified template elsewhere in the DOM.

We use the spinner while performing background operations and show snackbars as milestones are reached. Unfortunately since the snackbars are within the CDK overlay we can't manipulate their z-index to display above the spinner's overlay.

I'm still quite new to Angular so I don't know how to achieve this myself but I'm keen to help implement this feature.