Issue: Migrate ZuulFilter async execution from RxJava Observable to Java 8 CompletableFuture

Issue: Migrate ZuulFilter async execution from RxJava Observable to Java 8 CompletableFuture: a task in LegoFlow-SWE (Harbor dataset). We want to move to a standard Java 8 CompletableFuture for filter composition and asynchronous execution, while preserving backward compatibility for existing…

The task

We want to move to a standard Java 8 `CompletableFuture` for filter composition and asynchronous execution, while preserving backward compatibility for existing filters that already override `applyAsync`.

Part of Lego-X/LegoFlow-SWE.