Make all methods on DaprClient virtual

Hello, I've recently begun evaluation and usage of Dapr and I love it. This client library is also great, however I have had some difficulty in unit testing, specifically mocks. The DaprClient does not implement an interface and thus makes unit testing a little harder. I would like to see the DaprClient implement an interface, maybe IDaprClient, so that I can mock out things like InvokeMethodAsync and PublishEventAsync to return expected test data. Moq is the library I use for mocking.

I'd be happy to hear other suggested approaches, and while I haven't worked on any GitHub projects before I would be happy to make this addition if it's deemed worthwhile. Thanks for reading.