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.

首先跟踪 stackablectl 如何处理堆栈安装和演示选择，尤其是安装 Airflow DAGs 和 Jupyter notebooks 的路径。定义一种机制，使演示能够启用条件资源，而官方堆栈忽略这些资源；或者确定单独的演示堆栈将如何工作；完成标准是：不必要的演示资源不再默认安装。