Implement Domain based locale detection

[cmaerz]

Is your feature request related to a problem? Please describe.
As mentioned in #1127 domain based locales are missing as a supported feature. In my current project I need the functionality. https://nextjs.org/docs/advanced-features/i18n-routing#domain-routing

Describe the solution you'd like
As my research shows the real hostname is forwarded till the _app.js file, but the domains confgured in next.config.js are not matched properly. I'm not really sure why it's not working.
Does anyone have hints for me? I would try to solve it myself, but I have no idea where to start.

[dphang]

Yes sorry, this one is not implemented yet. If there is interest I can do it.

I think we will have to update the router around here to add the correct locale based on domain:

serverless-next.js/packages/libs/core/src/route/index.ts

Line 153 in 21ac575

const uri = normalise(req.uri, routesManifest);

(similar to how we check hostname in the domain redirect).

[dphang]

This was added in the latest alpha versions, I think it should be working now?