Add ability to extend database session and use in session callback

Description 📓

Currently, as you can see here, only a subset of the session (from the database) is passed to the session callback.

Pasting here as well:

// packages/core/src/lib/routes/session.ts
const sessionPayload = await callbacks.session({
        // By default, only exposes a limited subset of information to the client
        // as needed for presentation purposes (e.g. "you are logged in as...").
        session: {
          user: {
            name: user.name,
            email: user.email,
            image: user.image,
          },
          expires: session.expires.toISOString(),
        },
        user,
      })

If I want to extend that session to add additional properties and use them within the session callback, I need to go back to the database with the sessionToken (which is not available within the session call so I end up needing to do a custom initializer to pass it in). It's not very efficient to do the extra database query, and the DX is a pain.

I would love to have the ability to extend which session fields we pass (maybe using a whitelist?), or better yet, we could just pass the session to the callback directly how we get it from the DB, and omit any unnecessary fields (like id for example).

How to reproduce ☕️

N/A

Contributing 🙌🏽

Yes, I am willing to help implement this feature in a PR

Start with packages/core/src/lib/routes/session.ts, especially the database-session path and the callbacks.session call shown in the issue. Trace which session fields come from the database and define how the callback should receive additional fields while omitting unnecessary ones; done means the callback can use extended session data without an extra database query.