[bun dev] Automatically set `origin` - improve support for proxying Bun

Previously, when running Bun behind a reverse proxy, you had to pass an explicit `--origin` arg and it could only run behind one proxy at a time.

Now, Bun automatically determines the origin from the request if possible. It reads `Forwarded`, `X-Forwarded-Proto`, `X-Forwarded-Host`, `Origin`, and lastly `Host`. If none are available, it falls back to the `--origin` CLI arg.

This change is important for usecases like Replit which shows multiple iframes in different origins.