chore: Set a default impl for toBuilder in ExternalAccountCredentials

Author: lqiu96

oauth2_http/java/com/google/auth/oauth2/ExternalAccountCredentials.java

@@ -683,7 +683,10 @@ private static boolean isValidUrl(String url) {
   }
 
   @Override
-  public abstract Builder toBuilder();
+  public Builder toBuilder() {
+    throw new IllegalStateException(
+        "A derived class of ExternalAccountCredentials should be used to create a Builder");
+  }
 
   /**
    * Encapsulates the service account impersonation options portion of the configuration for