Add ability to omit `Vary: Origin` header

Hello! Thank you to the authors and maintainers for this important package.

We have an issue where we would like the ability to _conditionally_ omit `Vary: Origin` as a header. In our use case, we have specific routes for which the responses does not vary based on the requesting origin, e.g. a route for vending static image content.

Without this functionality, we are unable to use Cloudflare Polish[^1], which only accepts `Vary: Accept-Encoding`[^2], short of effectively rewriting all the `cors` code minus the `Vary: Origin` header.

Would this repository be able to add this functionality, or accept a patch to allow conditionally removing this header based on the requested routes?

Thank you again!

[^1]: https://developers.cloudflare.com/images/polish/
[^2]: https://developers.cloudflare.com/images/polish/cf-polished-statuses/