allow manual control of tasks

[kent-ps]

Describe the current behavior

Currently the only (official/documented) way to define a task is adding @task decorator to functions. For jobs that involve large numbers of long interwoven concurrent tasks, they are often wrongly nested together in the Prefect Cloud UI.

Describe the proposed behavior

I'd like to have the ability to:

• manually define a task's parent, so I can choose where to nest a task
• even better: manually create an empty task, which acts as a logical group of smaller tasks. This wrapping task will end when no more subtasks are manually added to it.

Example Use

def parse_sitemap(self, response, **cb_kwargs):
  group = new_task()
  ... 
  for listing in listings:
    yield Request(... cb_kwargs={'group_id' = group.id})

@staticmethod
def get_listing_parent():
    parameters = task_run.parameters
    group_id= parameters["cb_kwargs"].get('group_id')

    return group_id

@task(name="Parse listing", parent=get_listing_parent)
def parse_listing(self, response, **cb_kwargs):
  ...

Additional context

No response