Setup pass-through proxy

For gitlab-org/gitlab#439319 (closed)

How to test

1. Checkout this branch

2. Set up local interface

   gdk config set hostname gdk.test
   gdk reconfigure
   
   gdk restart

3. (optional) Create .dev.vars with the following:

   GITLAB_PROXY_HOST = "gdk.test:3000"

4. Start this router

   npx wrangler dev --env dev

5. Go to http://gdk.test:8787, you should see the sign in screen. Or the dashboard page if you already logged in.

6. Log in if not already

7. Try creating an issue in a project (POST request)

Screenshot