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.

首先追蹤 stackablectl 如何處理堆疊安裝與示範選擇，尤其是安裝 Airflow DAGs 和 Jupyter notebooks 的路徑。定義一種機制，讓示範能夠啟用條件式資源，而官方堆疊略過這些資源；或確定獨立的示範堆疊將如何運作；完成標準是：不必要的示範資源不再預設安裝。