Decouple generated asset classes by introducing flutter_gen_interface package

[Mairramer]

feat: add flutter_gen_interface package

What does this change?

This PR introduces the flutter_gen_interface package, extracting the AssetGenImage, SvgGenImage, and AssetGenImageAnimation classes into a shared, standalone dependency.

Motivation and Context:
In modular Flutter architectures (Monorepos, Design Systems), generating these classes inline within each package causes type conflicts, as each package has its own isolated version of AssetGenImage. By importing these base classes from flutter_gen_interface, all packages now share the same inheritance tree, allowing strongly-typed assets to be passed seamlessly across package boundaries.

Fixes #759

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

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  • Ensure the tests (melos run test)
  • Ensure the analyzer and formatter pass (melos run format to automatically apply formatting)
• Appropriate docs were updated (if necessary)