Add ParameterizedTypeReference to ModifyRequestBodyGatewayFilterFactory

[wuxudong]

Is your feature request related to a problem? Please describe.
ModifyRequestBodyGatewayFilterFactory only support Class inClass, it is not compatible with generic type.
expected code:

builder.routes()
  .route("test_route", r -> r.path("/**").filters(f ->
          f.modifyRequestBody(
                  new ParameterizedTypeReference<MultiValueMap<String, String>>() {},
                  new ParameterizedTypeReference<MultiValueMap<String, String>>() {},
                  (RewriteFunction<MultiValueMap<String, String>, MultiValueMap<String, String>>) (exchange, body) -> Mono.just(body))))

Describe the solution you'd like
serverRequest.bodyToMono support both class and ParameterizedTypeReference.
I think ModifyRequestBodyGatewayFilterFactory should support both of them too.

ServerRequest

	/**
	 * Extract the body to a {@code Mono}.
	 * @param elementClass the class of element in the {@code Mono}
	 * @param <T> the element type
	 * @return the body as a mono
	 */
	<T> Mono<T> bodyToMono(Class<? extends T> elementClass);

	/**
	 * Extract the body to a {@code Mono}.
	 * @param typeReference a type reference describing the expected response request type
	 * @param <T> the element type
	 * @return a mono containing the body of the given type {@code T}
	 */
	<T> Mono<T> bodyToMono(ParameterizedTypeReference<T> typeReference);

Describe alternatives you've considered
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Additional context
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