Add allow_failure annotation to allow failed runs to be passed downstream

[zanieb]

Closes #7098

Example

@task
def fails():
    raise ValueError("Fail task!")

@task
def bar(y):
    return y

@flow
def test_flow():
    f = fails.submit()

    # Previously, `bar` would fail with a `NotReady` state
    # Now, it will receive an exception
    b = bar.submit(allow_failure(f))
    assert isinstance(b.result(), ValueError)
    
    # Previously, `bar` would fail with a `NotReady` state
    # Now, it will run after the upstream completes
    b = bar.submit(1, wait_for=[allow_failure(f)])
    assert b.result() == 1

Checklist

• 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.
• This pull request includes tests or only affects documentation.
• This pull request includes a label categorizing the change e.g. fix, feature, enhancement

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[zanieb]

I'd like to add some documentation before merging, but otherwise this is pretty straightforward.

[EmilRex]

🚀

[zanieb]

@tpdorsey do you know where documentation would best fit for this?

[tpdorsey]

@madkinsz I think Tasks is the right place for this

[bunchesofdonald]

Thank you for updating this.

[zanieb]

There's no wait_for documentation right now. I can write it but I'd like to get this merged and do it as a follow-up.