Implement `Vary` and `Key` HTTP headers

[Syndesi]

See also https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Vary and https://greenbytes.de/tech/webdav/draft-ietf-httpbis-key-latest.html .

[Syndesi]

The Vary and Key headers will not be implemented at this time, as they are not required. Currently only two (and soon 3) headers will have an effect on the response:

• Authorization: Returns the response if access is granted, otherwise error codes.
• If-Match and If-None-Match: Might return the error code 412.

According to the internet and RFC, both cases are implied in normal cache mechanisms, see also:

• https://stackoverflow.com/a/53884651
• https://datatracker.ietf.org/doc/html/rfc7234#section-3