Add support for HTTP/3

[pmeenan]

The Priority HTTP header from HTTP Extensible Priorities (RFC 9218) may not be implemented for HTTP/1 and HTTP/2 and testing fetch interop would benefit from having access to a HTTP/3 server for web platform tests (see fetch issue here).

Chrome currently sends the Priority header in HTTP/2 and HTTP/3 but, as far as I can tell, Firefox and Safari send it only for HTTP/3 (which is what it was originally designed for).

It looks like we are likely to have fetch only send the header if there isn't already a user-specified value for the same header and it would be useful to test that the header doesn't get overwritten in all 3 HTTP versions.

It looks like there is some initial exploration on the topic in the quic doc in the wpt rfcs repository: https://github.com/web-platform-tests/rfcs/blob/master/rfcs/quic.md