improved UA Http Emulation of Priority header

[GlenDC]

More information about this header can be found in this recent talk: https://fosdem.org/2025/schedule/event/fosdem-2025-4852-how-browsers-really-load-web-pages/

The gist is that depending on the resource and its context it can change in value. The good news is that:

• we already work with different initiators, which will at least already have a difference between navigate and fetch requests
• one can always overwrite the header by setting the value given we won't consider this a locked base header

But still. Would need some research, but I'm pretty certain we can do a better job here in auto emulating it based on the given resource.

Might still turn out to be a won't do, and either for now low prio. But It's something we should revisit in 0.3.

Not available to pick up by anyone for now, as it is anyway blocked on the fact that we don't even have our first draft of UA emulation ready to begin with.