Allow Flow States to be determined by tasks other than the terminal tasks about prefect

[cicdw]

Brain-monsoon for the name of this kwarg:

• reference_tasks
• characteristic_tasks
• indicative_tasks
• representative_tasks
• diagnostic_tasks
• marked_tasks

[jlowin]

what about very explicit along the lines of:

prefect.Flow(set_flow_state_from=[x, y])

[jlowin]

Brings up another point: often, Flows will be instantiated before tasks exist. This option will probably be set post-init (even though it is an init option)... wonder if that changes things.

with Flow() as flow:
    x = x()
    y = y()

# ?????
flow.indicative_tasks = [x, y]
flow.set_flow_state_from = [x, y]
flow.set_flow_state_from(x, y)