Implement Task Macros to Chain Routines

[ditsuke]

Is your feature request related to a problem? Please describe.

At the moment, a Task is associated with only a single (task) routine. Looking at a task's anatomy, each has a "type" which is really a unique string corresponding to one of the task routines offered by task plugins.

As a user, there's no clean way for me to create tasks that consist of multiple routines. An example of such a task would be to backup the site before an update or to define a complex cleanup task.

Describe the solution you'd like

This leaves potential for improvement. For example a task could be associated with multiple routines in order. Then each routine execution could be a "stage" in the execution of a complete task. This would enable the creation of more complex tasks without the need to define a routine for each composite or using hacks like baking in routine associativity, which is generally not a flexible and extensible approach.

In the admin back-end, such a macro could be created perhaps by taking inspiration from Stages in a Workflow.

Additional context

None yet.