Consider dropping custom CorsMiddleware in favor of mezzio/mezzio-cors package

[acelaya]

It will first require adding some API tests that check the requests headers are handled as expected. Then, add the new package, and verify everything continues working as before.

https://github.com/mezzio/mezzio-cors

[acelaya]

After some testing with the component, two main issues arised.

• It tries to match routes internally, which doesn't work as expected when the middleware is piped for subpaths only. I'll consider reporting this.
• It doesn't allow to set header values dynamically, which is used by shlink in some scenarios.

It has allowed me to understand some edge cases better, which in turn allows to improve project's custom middleware.