feat(cdk/menu): Add backdrop support and maybe able to customize all overlay options in CDK Menu

[kdb13]

Feature Description

I think the CDK Menu Trigger should provide an ability to set a backdrop in the overlay and also other OverlayConfig options.

Use Case

This lets us create a custom Menu closer to the Mat Menu which already supports a transparent backdrop.

[crisbeto]

For context, we went without the backdrop because users preferred not to have the extra click to close the menu when clicking away. I'm leaving the issue open to see if there's any interest in this feature.

[kdb13]

@crisbeto Yeah, that's a valid point. But in some cases we actually need the backdrop, like when the menu overlay is displayed over other interactive elements like a link or a button. In that case we need to be careful to click on a non-interactive space outside the menu, to close the menu and not trigger those links or buttons.

With a backdrop, this won't be a problem (even if we need to click twice).