Add gRPC interop tests to our PR test suite

[JunTaoLuo]

Investigate adding gRPC interop tests. These should not be mandatory.

[Pilchie]

@JamesNK @JunTaoLuo I'm guessing this isn't happening for Preview 9 at this point?

[JamesNK]

@JunTaoLuo You're the best out of the two of us to do this one. Do you think you'll have time for it in the next couple of weeks?