Extend CORSPolicy to allow for custom headers

**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 类，并审查它当前如何构建 CORS 响应标头。使用链接的 CORS 规范指导来定义预期的逗号分隔的 Access-Control-Expose-Headers 值；完成标准是可链式调用的 exposeHeader 方法接受一个标头或模式，并正确生成该标头。

由索引模型根据 Issue 内容生成。