xDS conformance test suite

It would be helpful to control plane implementors and client implementations to have an xDS conformance test suite that is independent of Envoy. I think this would largely look like some scripts (Python or Go) that create xDS gRPC connections and exercise some xDS exchanges.

A priority area here is delta xDS, which control plane implementors consider to be challenging to verify.

I think we could separate this work into two parts; building out the basic test infra and then building out an increasingly more complete test suite.

@snowp @howardjohn @derekargueta