Add JSON validator sample for WinRT

[kennykerr]

This sample follows on from #2815 (article) and demonstrates how you can implement a WinRT-style API in Rust.

This sample component implements the following IDL:

namespace Sample
{
    runtimeclass JsonValidator
    {
        JsonValidator(String schema);
        String Validate(String value);
    }
}

The main takeaways compared to the Win32-style API is that this requires dramatically less code overall and in particular very little unsafe code, greatly simplifying code reviews. The accompanying tests are also greatly simplified as many of the edge cases are handled automatically. This is all due to WinRT's safer type system translating into greater safety guarantees.

The generated bindings.rs are included in this PR for code gen validation and education but would not normally be included in production.