Adding origins whitelist support for CORS

Motivation:
Currently the CORS support only handles a single origin, or a wildcard
origin. This task should enhance Netty's CORS support to allow multiple
origins to be specified. Just being allowed to specify one origin is
particulary limiting when a site support both http and https for
example.

Modifications:
- Updated CorsConfig and its Builder to accept multiple origins.

Result:
Users are now able to configure multiple origins for CORS.

[https://github.com/netty/netty/issues/2346]