Make server origin configurable

[misl-smlz]

We have a problem with the server origin: When we run our astro application in aws with cloudfront, auth.js always gets the "inner origin" (api gateway url or lambda function url) and not the cloudfront url.

Looking at the nuxt auth middlewares, they make the server origin configurable. See for example nuxt-auth: getServerOrigin.

We need something similar to make astro work with auth-astro in aws. Any suggestions on where to integrate this? I'm willing to look into this if requested.