Ability to specify AUTH_URL in NextAuthConfig

[OSmall]

Goals

1. Have an optional url key in the NextAuthConfig interface.
2. The url key will act the same as the AUTH_URL environment variable.
3. Behave in a similar way as the secret option whereby it acts as though the AUTH_SECRET environment variable was set.

Non-Goals

No response

Background

I'm using 5.0.0-beta.9
I am using SST to deploy my stack and their recommended method of transferring environment variables is using their built-in Config object which is only available after the stack has been deployed and bound. I can pass in environment variables but it means I am going against their patterns. It would be useful if I could pass in the AUTH_URL variable as a config option in NextAuthConfig.

Proposal

Add a url option to NextAuthConfig as another method of defining AUTH_URL. Similar to how the secret option is another way to define AUTH_SECRET.