fix: add missing __init__.py

[KazuCocoa]

To fix packaging:

The count of files in 'appium' and 'build/lib/appium' were different.
'appium' has '{'/webdriver/extensions/flutter_integration/flutter_commands.py', '/webdriver/extensions/flutter_integration/scroll_directions.py', '/webdriver/extensions/flutter_integration/flutter_finder.py'}' files than build/lib/appium
Python files in 'build/lib/appium' may differ from 'appium'. Please make sure setup.py is configured properly.

[KazuCocoa]

I'll merge and run release script after this CI pass