Add ability to wait on or get results of groups of futures

[desertaxle]

Adds some syntactic sugar to get the results or wait on futures returned from .map. The implementation is very naive and will need some iterative improvements, but this makes it feel a big more natural to work with groups of futures.

With this PR, .map will return a PrefectFutureList, which walk and quacks like list, but also exposes .wait_for_all and .results methods. These methods can be used to wait for all futures in the PrefectFutureList or get their results.

Very open to push back on this interface, so please review with a critical eye.

Closes https://github.com/PrefectHQ/prefect/issues/14017

Example

Map a task and wait for all the returned futures:

from prefect import flow, task

@task
async def say_hello(name):
    print f"Hello {name}!"

@flow
def my_flow():
    futures = say_hello.map(["world", "Alex", "Sam", "Ned"])

    futures.wait()

if __name__ == "__main__":
    my_flow()

Map a task and get all the results:

from prefect import flow, task

@task
async def say_hello(name):
    return f"Hello {name}!"

@flow
def my_flow():
    futures = say_hello.map(["world", "Alex", "Sam", "Ned"])

    print(futures.result())

if __name__ == "__main__":
    my_flow()

Checklist

• [x] This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.
• [x] This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• [x] If this pull request adds new functionality, it includes unit tests that cover the changes
• [ ] If this pull request removes docs files, it includes redirect settings in mint.json.
• [ ] If this pull request adds functions or classes, it includes helpful docstrings.

[cicdw]

Should probably be in a separate PR, but I think we'd also benefit from a wait(*all_my_futures) utility for waiting on arbitrary groups of futures (not necessarily mapped). That being said, I like this interface for mapped futures!

[desertaxle]

I think we'd also benefit from a wait(*all_my_futures) utility for waiting on arbitrary groups of futures

Agreed! I'd like us to have wait and as_completed utilities that mirror those in concurrent.futures in the stdlib. I'll add issues to make sure we remember.