Add IModalSharedContext interface and make it public

[stsrki]

Closes #4660

[David-Moreira]

I guess the real problem here, is that we were using DI for an internal member. But I guess it's necessary to easily get the benefit of scoping it. So from a api design perspective maybe we shouldn't have to change it to public on purpose.
But I guess it's the easier route and there's no real harm on it, so LGTM.

[stsrki]

I guess the real problem here, is that we were using DI for an internal member. But I guess it's necessary to easily get the benefit of scoping it. So from a api design perspective maybe we shouldn't have to change it to public on purpose. But I guess it's the easier route and there's no real harm on it, so LGTM.

This should get easier once we do the Blazorise packages for testing.