Spring Support @PathVariable in Functional Closures Returned by WebFlux Endpoints

Allow @PathVariable, @RequestParam, and similar annotations on parameters of functional closures (e.g., Consumer, Function) returned by Spring WebFlux endpoints. Motivation: Spring WebFlux’s functional endpoints lack support for annotating closure parameters with @PathVariable. This forces developers to pass parameters explicitly, making code less concise. For example:

@PutMapping("decrement/{word}")
public Consumer<@PathVariable("word") String> adjustVocabDown(@PathVariable("langId") Long langId) {
    return word -> languageService.propVocabularyFreq(langId, changeFrequency.apply(decrement)).accept(word);
}

Supporting annotations in closures would simplify endpoint definitions and align with Spring MVC’s annotation model.

Proposed Solution: Enable Spring (WebFlux, Web) to process @PathVariable, @RequestParam, and @RequestBody on closure type parameters. Bind request data (e.g., path variables) to these parameters automatically, similar to annotated controllers.