proxy: vibe-coded reverse proxy

[wryun]

I will not be at all offended if you close this PR immediately, and my apologies for the noise.

Useful when serving things like ttyd/mediamtx from LuCI via https. If nothing else, browsers don't like including non-https iframes etc.

There are tests in tests/ that pass, and I've done some brief testing of this in front of ttyd and mediamtx, but I haven't carefully reviewed the code.

For the OpenWrt uhttpd init script, you'd want something like:

       config_get reverse_proxy "$cfg" reverse_proxy
       for rp in $reverse_proxy; do
                procd_append_param command -Y "$rp"
       done

You can then do:

        list reverse_proxy '/mediamtx/hls=127.0.0.1:8888/'
        list reverse_proxy '/mediamtx/webrtc=127.0.0.1:8889/'
        list reverse_proxy '/ttyd=127.0.0.1:7681/'

Which would lead to something like:

root@pc-q35-ich9-2009-3338:~# ps ww | grep uhttpd
 9288 root      6768 S    /usr/sbin/uhttpd -f -h /www -r pc-q35-ich9-2009-3338 -x /cgi-bin -u /ubus -t 300 -T 30 -k 20 -A 1 -n 3 -N 100 -R -Y /mediamtx/hls=127.0.0.1:8888/ -Y /mediamtx/webrtc=127.0.0.1:8889/ -Y /ttyd=127.0.0.1:7681/ -p 0.0.0.0:80 -p [::]:80 -C /etc/uhttpd.crt -K /etc/uhttpd.key -s 0.0.0.0:443 -s [::]:443