gae-interop-testing: add GAE interop tests (#3535)

See `gae-interop-testing/README.md` for details on how to run the tests.
diff --git a/build.gradle b/build.gradle
index ce47acb..b697be3 100644
--- a/build.gradle
+++ b/build.gradle
@@ -318,6 +318,7 @@
         repository(url: stagingUrl, configureAuth)
         snapshotRepository(url: 'https://oss.sonatype.org/content/repositories/snapshots/', configureAuth)
     }
+    uploadArchives.onlyIf { !name.contains("grpc-gae-interop-testing") }
 
     [
       install.repositories.mavenInstaller,