Make User-Agent request header match RFP's navigator.userAgent

Currently there is a mismatch between the two user agents, for example on my Linux machine:

navigator: Mozilla/5.0 (X11; Linux x86_64; rv:109.0) Gecko/20100101 Firefox/116.0
HTTP header: Mozilla/5.0 (Windows NT 10.0; rv:109.0) Gecko/20100101 Firefox/116.0

I understand there are implications with passive fingerprinting, and when in strict mode this mismatch makes more sense than not, but it also causes a lot of breakage. me and @thorin have reported this to Mozilla for months now, I'll hunt the ticket.

so in a similar fashion to tor-browser!742 (merged) we could have this discussion about improving usability, it would especially make sense once upstreamed to Firefox where it could be the default behavior for RFP users while in Tor Browser it could stay as it currently is. a dedicated pref would make sense, then @ruihildt can decide what's best for Mullvad Browser.

@thorin suggets the name privacy.resistFingerprinting.match_userAgent.