feat(tools): Add 'gotest' program to demonstrate pkgs.buildGo

This is a tiny program that does nothing but exists to demonstrate
pkgs.buildGo by building a program that depends on a local library as
well as a protobuf definition.

tools/gotest/test.proto

@@ -0,0 +1,9 @@
+syntax = "proto3";
+package someproto;
+
+import "google/protobuf/timestamp.proto";
+
+message Person {
+  string name = 1;
+  google.protobuf.Timestamp last_updated = 2;
+}