Extend SubTasksStrategy to manage multiple configurable task / sub-task strategy mechanisms

Currently we have the possibility of setting just one SubTaskStrategy for a given task, when in some cases it might make sense to have multiple (i.e. for the current implementation status, it could make sense to skip the children when the parent is skipped AND to end the parent when all chldren end)

Also, it would be useful to have the possibility of having a plugable discovery mechanism for sub task strategies, where the sub task strategy itself defines what to do with the task, its children, and the parent task, instead of kipping that logic only in ProcessSubTaskCommand

For this, I'm proposing the following:

1. the task in the model should change from having a single SubTaskStrategy to possibly multiple sub task strategies (keeping the first one on the list as optional first element would maintain backward compatibility)
2. To change SubTaskStrategy from an enum to an interface, with a apply() method that receives the parentTask, task and subTasks to decide how to apply changes to them
3. A new Environment variable to plug extra sub task strategies (other than the 3 existing ones that apply today)
4. Change ProcessSubTaskCommand to iterate all sub task strategies to apply(...) all cases