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.

まず、stackablectl でスタックのインストールとデモの選択がどのように処理されているか、特に Airflow DAGs と Jupyter notebooks をインストールする経路を追跡します。公式スタックではそれらを省略しつつ、デモで条件付きリソースを有効化できる仕組みを定義するか、分離されたデモスタックがどのように機能するかを判断します。不要なデモリソースがデフォルトでインストールされなくなれば完了です。