Azure DTDL & IoT Plug and Play Support

There were recent announcements regarding Azure's new Digital Twin Definition Language (DTDL) and [IoT Plug and Play](https://github.com/Azure/IoTPlugandPlay) at Build 2019. We should review and collaborate with that team as this repo and APIs could "plug and play" nicely with each other.

There are scenarios that can help with System.Device.* tooling and code generation for device bindings. This would also be extremely helpful to better define bindings as this has been a huge gap/struggle trying to enforce consistent standards between contributors. Device GPIO pin assignments, I2C/SPI settings, and register definitions are just a few examples where it would benefit. In addition, I could also see these types of DTDL docs being used for [external configuration in containers](https://github.com/dotnet/iot/issues/8).

I already created a few issues on the [IoTPlugAndPlay repo](https://github.com/Azure/IoTPlugandPlay/issues) as I see some limitations related to DTDL. For example, it doesn't currently [support unsigned data types](https://github.com/Azure/IoTPlugandPlay/issues/8).

It might be worth doing an exercise of defining a few of our current bindings using DTDL and see what issues/ideas/innovation can be derived.

Thoughts?

//cc @joperezr @joshfree @richlander