Upstream XDS interop tests proto changes to grpc-proto.

[creamsoup]

Even though the grpc-proto is not being really used. we should upstream new messages to grpc-proto.

https://github.com/grpc/grpc-java/blob/master/interop-testing/src/main/proto/grpc/testing/messages.proto

new messages: LoadBalancerStatsRequest, LoadBalancerStatsResponse