fix(ts): update `GetTokenParams` type definition to make `salt` optional

[simonecervini]

☕️ Reasoning

This PR updates the GetTokenParams type definition to make the salt property optional. The default value for salt was already in place, but the type definition needed adjustment to reflect this.

Now, external calls to getToken({ req, secret: process.env.AUTH_SECRET }) no longer require hardcoding "__Secure-authjs.session-token" or "authjs.session-token" for the salt value.

🧢 Checklist

• [ ] Documentation
• [ ] Tests
• [ ] Ready to be merged

🎫 Affected issues

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source