Show HTTP/3 support (for DoH3) in header response

Hi,

Currently HTTP header responses from dns.controld.com do not respond with the alt-svc: h3 header as defined by the HTTP/3 spec. As a result, DoH3 supported clients such as Firefox will not not use DoH3. Sending this header will automatically allow supported clients to use DoH3.

Example of this working can be seen by querying doh3.dns.nextdns.io.

Best