Add Redwood Self-Hosted Database Authentication example

[dthyresson]

Currently, the authentication provider examples are all third-party.

But, redwoodJS also supports a Self-Hosted Database Authentication option: https://redwoodjs.com/docs/authentication#self-hosted-auth-installation-and-setup

This issue is to implement "dbAuth" provider and the example forms, code samples, etc.

The playground-auth app already has a Supabase Postgres database available and is configured in Netlify on deploy, so any migrations and setup should be able to be run -- even if implementing the database locally.