Add native library sample

[jkotas]

Fixes #4987 and #6644

[tonerdo]

Ah, thanks! Was trying to come up with some fancy real-world use case but I can save that for a blog post. Someone already found a use case with Rust https://medium.com/@chyyran/calling-c-natively-from-rust-1f92c506289d

[tonerdo]

Nit: Building Native Libraries with CoreRT

[filipnavara]

Thanks for the work and documentation (especially to @tonerdo, whose PR served us as a good example). We have a use case for this in our application that we previously implemented using NetFX and assembly rewriting. We implement an email client that provides the standardized Simple MAPI interface. For that we need to have a DLL with specific exported functions that can be loaded by any application. Using NetFX was painful because of lack of official support and conflicts between different runtime versions loaded into single process (unlike executable programs we cannot control it with manifests).