Test JSON-Schema generation on multiple packages

[tcompa]

Given the planned activity on JSON Schema generation (see e.g. #755 and #756), I think it'd be useful to quickly assess how a given change would affect other Fractal-compatible packages.

I propose we add a GitHub action that acts on a few selected packages. For each one of them we would

1. git clone the repo (required metadata repository_url, e.g. https://github.com/fmi-basel/gliberal-scMultipleX)
2. Install the package (required metadata cmd_install, e.g. pip install -e .[extras], poetry install --all-extras, ...)
3. Run the manifest-creation script (required metadata cmd_create_manifest, e.g. python src/dev/create_manifest.py)
4. Compare the new manifest with the old manifest (this can be done with git diff, or even more easily with a Python script.. the required metadata could then be manifest_path and/or current_manifest_url).

The goal here would not be to block updates of the fractal-tasks-core dev tools. Rather, we should be aware of which changes may be breaking for a different package (as in "the manifest creation will fail") or somehow relevant (as in "the new manifest differs from the old one"), and we can also warn maintainers in advance.

[jluethi]

Great idea!

[jluethi]

Let's do this for scmultiplex, APX & fractal-helper tasks