Extend CORSPolicy to allow for custom headers

還沒有人認領這個 Issue。

**Is your feature request related to a problem? Please describe.**
By default CORS blocks access to all headers except `content-type`. Custom headers need to be exposed using the header `Access-Control-Expose-Headers`.

**Describe the solution you'd like**
The CORSPolicy class should have a chainable method called `exposeHeader` which will take the header/pattern to be further used by building a comma seperated header value for `Access-Control-Expose-Headers` according to the spec.

**Describe alternatives you've considered**
I've implemented a simple RequestInterceptor which sets this header manually.

**Additional context**
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Headers

定位 CORSPolicy 類別，並檢視它目前如何建構 CORS 回應標頭。使用連結的 CORS 規範指引來定義預期的逗號分隔 Access-Control-Expose-Headers 值；完成的定義是可鏈式呼叫的 exposeHeader 方法接受標頭或模式，並正確產生該標頭。

由索引模型根據 Issue 內容生成。