Add RequestPredicate Visitor to WebFlux.fn

This commit introduces RequestPredicates.Visitor, an implementation of
the visitor pattern for RequestPredicates.

Issue: SPR-17322

Author: poutsma

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RequestPredicate.java

@@ -84,4 +84,15 @@ default Optional<ServerRequest> nest(ServerRequest request) {
 		return (test(request) ? Optional.of(request) : Optional.empty());
 	}
 
+	/**
+	 * Accept the given visitor. Default implementation calls
+	 * {@link RequestPredicates.Visitor#unknown(RequestPredicate)}; composed {@code RequestPredicate}
+	 * implementations are expected to call {@code accept} for all components that make up this
+	 * request predicate.
+	 * @param visitor the visitor to accept
+	 */
+	default void accept(RequestPredicates.Visitor visitor) {
+		visitor.unknown(this);
+	}
+
 }