Make installation of stack steps conditional

Description

Some demos have requirements that can only be installed at stack installation time. Examples are: airflow dags, jupyter notebooks. These cannot be installed by the demo it's self because that's too late.

Currently some stacks install resources that only make sense as part of demos. This can be confusing and leads to stack pollution with unnecessary steps and resources.

Proposed solution

Introduce some mechanism that allows stacks to create resources conditionally.

In the examples above, stacks should install airflow dags and jupyter notebooks conditionally. The corresponding demos would enable the condition while the stacks would not.

Alternatively, demo stacks could be separated from the "official" stacks.

Commencez par retracer la manière dont l’installation des stacks et la sélection des demos sont gérées dans stackablectl, en particulier les chemins qui installent des Airflow DAGs et des Jupyter notebooks. Définissez un mécanisme permettant aux demos d’activer des ressources conditionnelles tandis que les stacks officiels les omettent, ou déterminez comment fonctionneraient des stacks de demo séparés ; le travail est terminé lorsque les ressources de demo inutiles ne sont plus installées par défaut.