test: add dummy method to auth and use it in gax to test CI

Author: blakeli0

google-auth-library-java/credentials/java/com/google/auth/Credentials.java

@@ -194,4 +194,9 @@ protected final void blockingGetToCallback(URI uri, RequestMetadataCallback call
    * @throws IOException if there was an error getting up-to-date access.
    */
   public abstract void refresh() throws IOException;
+
+  /** Dummy method to test CI recognition of auth changes. */
+  public String getTestString() {
+    return "test";
+  }
 }

sdk-platform-java/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/GrpcCallContext.java

@@ -729,6 +729,9 @@ public <T> T getOption(Key<T> key) {
   @InternalApi
   public void validateUniverseDomain() {
     try {
+      if (credentials != null) {
+        credentials.getTestString();
+      }
       endpointContext.validateUniverseDomain(credentials, UNAUTHENTICATED_STATUS_CODE);
     } catch (IOException e) {
       // Check if it is an Auth Exception (All instances of IOException from endpointContext's