Spring Set BEST_MATCHING_PATTERN_ATTRIBUTE on requests with WebMvc.fn

The HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE is used by Spring Boot to instrument the web framework and tag request metrics with the path pattern that matched for the current request.

WebFlux.fn extracts that information and adds it as a request attribute in its RouterFunctionMapping, but the WebMvc.fn variant does not. We should align both implementations.