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.

Comece rastreando como a instalação de stacks e a seleção de demos são tratadas no stackablectl, especialmente os caminhos que instalam Airflow DAGs e Jupyter notebooks. Defina um mecanismo para que as demos possam habilitar recursos condicionais enquanto os stacks oficiais os omitem, ou determine como stacks de demo separados funcionariam; considera-se concluído quando os recursos de demo desnecessários não forem mais instalados por padrão.