Title: [Feature Request] First-Party, Native CORS and Security Headers

[jishjitsu]

Hey everyone, I recently started using Rocket and I'm really interested in contributing. I noticed the commit activity has been a bit quiet over the past few months and I know the maintainers are likely swamped.

Before I dive into writing code, I was thinking about implementing a first-party CORS integration. I just wanted to ping the community to find out if a PR for this would be welcome right now, and realistically, if it's worth the time to build given the current review backlog? I'd love to help out, but want to make sure I'm aiming at something that has a chance of getting merged as the latest closed PR was on dec 20.

[the10thWiz]

I believe Shield addresses most your features. rocket_cors provides a pretty good implementation of CORS.