v2.2.0 Compatibility with new `fastapi-slim` and updated examples

Maintenance update, with minor changes (from #94)

• Updated examples with lifespan usage (instead of deprecated .init_app() method)
• Better typing for decorators
• Increase required Python version to >=3.8 (syncing with fastAPI)
• Adjust dependencies to work with new fastapi-slim package variant, available since fastapi>=0.111