UserSession cannot be type-safe

[Vap0r1ze]

After trying to fix #422 on my project, I found out that the UserSession interface has a [x: string]: unknown index signature which basically removes all type information from the interface.

Even in this repo's playground, it has strict typing for UserSession, nor does it have intellisense / autocomplete

auth.d.ts is merged with ./runtime/types

but no intellisense

and no type safety

The only non-breaking solution I can think of for this is:

• Provide an off-switch for this in the module's config
• Move the [x: string]: unknown index signature from runtime/types/session.ts to an optional runtime/types/extra-session-keys.d.ts that adds the signature by extending '#auth-utils' like a user-land auth.d.ts would
• Add runtime/types/extra-session-keys.d.ts with addTypeTemplate(..., { nuxt: true, nitro: true }) unless the off-switch config is set to false