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.

Empieza por rastrear cómo se gestionan la instalación de stacks y la selección de demos en stackablectl, especialmente las rutas que instalan Airflow DAGs y Jupyter notebooks. Define un mecanismo para que las demos puedan habilitar recursos condicionales mientras los stacks oficiales los omiten, o determina cómo funcionarían stacks de demo separados; se considerará terminado cuando los recursos de demo innecesarios ya no se instalen de forma predeterminada.