We are setting up fallback test based on TD. Currently the test client is compiled in google3, so we must run it in a container so that the client can have the GRTE dependency. However, container does not have `ip`, `iptables`, etc network command, so we plan to run the network command outside of the container. To do this, add a new flag `skipNetCmd` to skip network commands inside the test client.
This is a java analogue of the C++ test added in grpc/grpc#19623.
Proto changes were synced from grpc/grpc-proto
This client is suitable for testing fallback with any "grpclb" load-balanced service, but is particularly meant to implement a set of test cases described in an internal doc titled "DirectPath Cloud-to-Prod End-to-End Test Cases", section "gRPC DirectPath-to-CFE fallback".