Add extras to conda-forge feedstocks

[tcompa]

I just realized that conda packages do not have extras. This means that especially the fractal-tasks-core conda-forge package cannot install task dependencies (e.g. cellpose).

See for instance this issue and links therein. One relevant quote:

So e.g. we end up with some package "pack" on pip, with "pack[recommended]" installing the most typical dependencies, "pack[all]" installing lots of extra things, and "pack" installing only the bare minimum. On conda we maybe get "pack" installing the most typical dependencies, sometimes "pack-core" (a dependency of "pack") installing the bare minimum, and "pack-extras" installing the extra stuff.

There are multiple options to be explored, e.g. the use of outputs, or also

• [recipe-spec] Allow for arbitrary optional dependencies mamba-org/boa#245
• ENH: More powerful syntax for build variants & optional package-extras conda/conda#11053
• https://github.com/conda-forge/airflow-feedstock/blob/main/recipe/meta.yaml#L185
• Optional groups of dependencies conda/conda#7502
• Mirroring the pip sub-package (extras) dependency semantics conda/conda#2984