add test that excludes generating source for google protos

based on https://github.com/scalapb/ScalaPB/issues/1746 and https://github.com/apache/pekko-grpc/pull/371

One of the issues in #371 is that the sbt build for plugin-tester-scala started failing. It was a real issue and I have submitted a fix to ScalaPB. The thing is though: one of the protos we are testing imports a Google Timestamp proto. There is no need to generate classes for the Google Timestamp proto because protobuf-java jar has a class already that represents this data type. What we end up with in our integration test is that we generate a Scala class for this Timestamp type and other Google types. These classes will clash with the protobuf-java classes. The classes should have the same methods but these days, Java is trying to get us to stop splitting packages across jars for security reasons (eg JSR 376).

It would be good to see if we can successfully get ScalaPB not to generate these Google classes. See https://github.com/scalapb/ScalaPB/issues/1746#issuecomment-2387414076

Start with the plugin-tester-scala sbt integration test that imports a Google Timestamp proto, then review the related ScalaPB issue and Pekko gRPC pull request for the expected configuration or behavior. Done means the test confirms Google protobuf classes are not generated locally and the sbt build no longer produces conflicting Timestamp or other Google classes.

Written by the indexing model from the issue text.