feat: proxy flag for additional cors headers

[afreakk]

When using non standard headers in proxy requests, it results in error like

Preflight aborted: headers 'traceparent' not allowed

With this change you can specify additional headers to allow.

Related Issue or Design Document

https://github.com/ory/cli/issues/261

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [ ] I have referenced an issue containing the design document if my change introduces a new feature.
• [ ] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added the necessary documentation within the code base (if appropriate).

Further comments