Expose Modal Window to support Dynamic Modal Resizing

[jebner]

We have a modal with a dynamic form. Based on a switch control, a large part of the form is displayed / hidden side-by-side with the existing form inside the same modal. Due to this, we would like to increase / decrease the modal size dynamically after it has been opened.

This could be easily achieved by setting the size / fullscreen properties on the NgbModalWindow.
Unfortunately, the NgbModalWindow is not exposed, so we used a little hack to access it: (ngbModalRef as any)._windowCmptRef.instance;

We see a few ways to add support for this:

1. Expose the NgbModalWindow class, such that the NgbModalWindow can be injected into a child component (this is the technical approach we sort of achieve with the hack above)
2. Add a public getter or field modalWindow on the NgbModalRef which provides _windowCmptRef.instance. This could return a custom-tailored interface type that only exposes what is reasonable to add to the public API

Or is there another way how we can achieve that?
Maybe we can help/provide a PR if we see a reasonable chance this would get integrated. But as it widens the public API I we would like to discuss first.

[maxokorokov]

Related to #4281