fix latest CI breakages by forking code from grpc-go's interop/testing

Also moves testing package to internal/testing

internal/testing/example2.proto

@@ -0,0 +1,8 @@
+syntax = "proto3";
+
+import "google/protobuf/any.proto";
+
+message Extension {
+    uint64 id = 1;
+    google.protobuf.Any data = 2;
+}