网关限流异常类ZuulBlockFallbackProvider中返回信息限定了BlockResponse，能不能支持自定义

## Issue Description

Type: *bug report* or *feature request*

### Describe what happened (or what feature you want)

网关限流异常类 ZuulBlockFallbackProvider 中返回信息限定只能返回 BlockResponse。我们期望用户可以真正自定义网关限流之后的处理逻辑，像 Web adapter 一样，可以实现跳转、自定义返回等。

Start by reading the ZuulBlockFallbackProvider and BlockResponse classes named in the issue, then trace how gateway rate-limit fallback responses are handled. Clarify the extension point needed for custom redirects or responses and define tests that demonstrate the supported behavior.

Written by the indexing model from the issue text.