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

Localiza la clase CORSPolicy y revisa cómo construye actualmente las cabeceras de respuesta CORS. Usa las indicaciones de la especificación CORS enlazada para definir el valor esperado, separado por comas, de Access-Control-Expose-Headers; se considera terminado cuando un método exposeHeader encadenable acepta una cabecera o un patrón y produce correctamente esa cabecera.

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