Spike: Supporting Authentication in the template

[brandon-lent]

Context

Many government projects require integration with an identity provider. This has been a reoccurring conversation with the platform team on how we should support authentication in the next.js application.

Goal

Create an ADR that:

1. Provides a recommendation on why we should use NextAuth in the template application
2. Outline pros/cons
3. Discuss alternatives
4. Get approval from template-application-nextjs code owners