Extract interface for ModuleClient, DeviceClient classes

[fgheysels]

I think it would be great if there would exist an IModuleClient and IDeviceClient interface, which would then be implemented by the ModuleClient and DeviceClient classes.
This would allow developers to easily test code / classes that have a dependency on those classes. Instead of directly injecting ModuleClient, one could inject IModuleClient and pass in a stub or mock in unit-tests.

[drwill-ms]

We're working on a v2 of the SDKs and are enabling unit testable device clients.