Completed interop spec, as well as code for c++

Makefile

build.yaml

doc/interop-test-descriptions.md

src/proto/grpc/testing/messages.proto

test/cpp/interop/client.cc

test/cpp/interop/interop_client.cc

test/cpp/interop/interop_client.h

test/cpp/interop/interop_server.cc

test/cpp/interop/rnd.dat

test/cpp/interop/server_helper.cc

test/cpp/interop/server_helper.h

tools/run_tests/sources_and_headers.json

...s/vcxproj/interop_server_main/interop_server_main.vcxproj

...j/interop_server_main/interop_server_main.vcxproj.filters