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.

Inizia tracciando come vengono gestite l’installazione degli stack e la selezione delle demo in stackablectl, in particolare i percorsi che installano Airflow DAGs e Jupyter notebooks. Definisci un meccanismo che consenta alle demo di abilitare risorse condizionali mentre gli stack ufficiali le omettono, oppure determina come funzionerebbero stack demo separati; il lavoro è completato quando le risorse demo non necessarie non vengono più installate per impostazione predefinita.