chore: add request.headers to /dev/debug

[hi-ogawa]

For #337, I was curious what vercel/aws-lambda puts in x-forwarded-??? headers.

$ curl -s https://ytsub-v3-hiro18181-hiogawa.vercel.app/dev/debug | jq -c '.["request.headers"] | .[]'
["accept","*/*"]
["connection","close"]
["forwarded","for=36.13.100.141;host=ytsub-v3-hiro18181-hiogawa.vercel.app;proto=https;sig=0QmVhcmVyIDg0NGNjMThlNjVkMDc3MmI5MmQ2M2IzMjY1Zjg3MDg5YjllYmVhNGI5OGE0M2FkNzUwMmRlYTEyNmJiOWY1NDc=;exp=1682937068"]
["host","ytsub-v3-hiro18181-hiogawa.vercel.app"]
["user-agent","curl/8.0.1"]
["x-forwarded-for","36.13.100.141"]
["x-forwarded-host","ytsub-v3-hiro18181-hiogawa.vercel.app"]
["x-forwarded-proto","https"]
["x-real-ip","36.13.100.141"]
["x-vercel-deployment-url","ytsub-v3-hiro18181-ixzb64l75-hiogawa.vercel.app"]
["x-vercel-forwarded-for","36.13.100.141"]
["x-vercel-id","hnd1::8pfmm-1682936768012-55f614ccd78f"]
["x-vercel-ip-city","Yokohama"]
["x-vercel-ip-country","JP"]
["x-vercel-ip-country-region","14"]
["x-vercel-ip-latitude","35.4343"]
["x-vercel-ip-longitude","139.6466"]
["x-vercel-ip-timezone","Asia/Tokyo"]
["x-vercel-proxied-for","36.13.100.141"]
["x-vercel-proxy-signature","Bearer 844cc18e65d0772b92d63b3265f87089b9ebea4b98a43ad7502dea126bb9f547"]
["x-vercel-proxy-signature-ts","1682937068"]