Move BindingAddress to public namespace

[davidfowl]

I'm not sure why this was made pubternal instead of public and why we're even using pubternal APIs across assemblies. We should fix this and BindingAddress should be public.

PS: This will break https://github.com/aspnet/BasicMiddleware/blob/726035e3fb8c4ee31a32522c16ff73278b9b13e7/src/Microsoft.AspNetCore.HttpsPolicy/HttpsRedirectionMiddleware.cs#L124, I'm not sure where else we use it.

/cc @Tratcher @jkotalik

[analogrelay]

We should actually just look at making it internal.

[Tratcher]

By which we mean shared source.

[analogrelay]

Yes, as a result of making it internal we'll need to shared-source it to "share" it properly.

[davidfowl]

I'd like to internal all the things as part of preview7 (or make it public why it think it should be public)