Add support for generator tasks

[jlowin]

This PR leverages #13793 to add native support for generator tasks to the new engine, both sync and async. In Prefect 2.x you can decorate generators as tasks, but they are not preserved as generators. Instead, when the task is called the generator is fully consumed and the resulting list is returned as the task's result. This PR preserves true generator semantics -- a task generator must be iterated over to produce values incrementally. The same goes for async generators.

Generator tasks inherit all properties of regular tasks, including retries and timeouts.

This PR also enhances dependency tracking to automatically mark generators as "parent" tasks of any tasks that consume the values they yield.

This PR does not add support for generator flows.

Here is a simple demo that shows generator yields interspaced with dependent task run executions:

import random
from prefect import task, flow

@task
def gen_numbers():
    for i in range(10):
        yield i
        print(f'yielded {i}')
    

@task
def process_number(n: int):
    print(f'got {n}')

@task
def show_status():
    print(random.choice(["👋", "🧐", "🤬", "💀"]))

@flow
def demo():
    for n in gen_numbers():
        process_number(n)
        show_status()


demo() 

got 0
🤬
yielded 0
got 1
🤬
yielded 1
got 2
💀
yielded 2
got 3
🧐
yielded 3
got 4
🤬
yielded 4
got 5
🤬
yielded 5
got 6
👋
yielded 6
got 7
🧐
yielded 7
got 8
💀
yielded 8
got 9
💀
yielded 9

[cicdw]

👏

[cicdw]

nice!

[zzstoatzz]

🔋

[chrisguidry]

It's the tests for me