v1: add flet-dev extensions to client

[ndonkoHenri]

Summary by Sourcery

Enable the flet_lottie extension in the Flutter client by adding its dependency, import, and initialization, and enhance asset model debugging.

New Features:

• Include flet_lottie extension in the client application with proper import and initialization.

Enhancements:

• Add a toString override to AssetSource for improved debug output.

Build:

• Uncomment and pin the flet_lottie dependency in pubspec.yaml to the v1 Git reference.