xds: specify "h2" as the alpn in xds creds (#4361)
xds: specify "h2" as the alpn in xds creds (#4361)


dfawley/grpc-go,menghanl/grpc-go,grpc/grpc-go,stanley-cheung/grpc-go,grpc/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go,dfawley/grpc-go