feat: Add support for custom refresh handling

[blumewas]

🔗 Linked issue

Issue #714

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [x] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The current behaviour for refreshing the users tokens does not provide many options for me as developer. As a developer I would like to be able to extend or customize this behaviour.

This PR:

• adds new types RefreshHandler and RefreshHandlerConfig
• adds the possibility to configure a custom RefreshHandler via refreshHandler? property to auth.session
• extracts the default refreshing behaviour to a separate file

📝 Checklist

• [x] I have linked an issue or discussion.
• [ ] I have added tests (if possible).
• [ ] I have updated the documentation accordingly.