Allow running flatpak-*-generator on update

[hadess]

When pocket-sync has a new release, I need to run flatpak-node-generator npm package-lock.json and flatpak-cargo-generator.py src-tauri/Cargo.lock manually to update the generated sources for both types of dependencies, even if flatpak-external-data-checker already updates the version.

Ideally, it would be possible to run helpers like flatpak-node-generator and flatpak-cargo-generator.py, but that would mean it would need to know how to:

• check out the upstream
• apply patches (as needed)
• run the generators with the correct arguments
• and then use those generated sources in the Flatpak update

The first 2 items become easier if we can ask flatpak-builder to do that for us.

[wvengen]

Similar need for https://github.com/flathub/org.artisan_scope.artisan which uses a custom script to update all dependencies (consisting of a number of steps in a specific order).