feat(passport): Use Rollup creds for OAuth logins when custom domain configured

[betimshahini]

We want to enable devs to leverage out OAuth client creds for the OAuth providers we support even in scenarios where they use their own custom domain.

To enable this, we'd need to leverage passport.rollup.id as a redirect-based proxy so that the session cookie is issued for the custom domain, while leveraging all the redirect logic that's currently in place for rollup.id.

High level this would work as follows:

• OAuth login options in custom domain Passport would point to /connect/(provider) routes in Passport under rollup.id
• Passport would set a property in the authenticator cookie with the custom domain and proceed with upstream OAuth authentication.
• On callback, introspect the cookie and retrieve the custom domain
• Redirect to custom domain Passport (route /connect/(provider)/callback) with query params from the callback to Passport under rollup.id

[szkl]

Passport would set a property in the authenticator cookie with the custom domain

The referer could be used to read the custom domain. Though, not sure how to validate it for custom domains only.