[sdk-platform-java] x-goog-api-client is incorrect when the customer uses shading

The current approach pulls the version from the jar manifest. Unfortunately this will break if the application builds an uber jar (the version will come from the uber jar rather then the generated client or gax). A more robust approach would to follow grpc's approach and embed it in a class or to use the maven-resources-plugin. I recently worked around this issue in the bigtable client:
https://github.com/googleapis/java-bigtable/pull/451