Support suspending stream consumers

[joefreeman]

This change allows stream consumers to use suspense. If a stream doesn't produce a value within the specified (optional) timeout, the execution suspends, and then gets woken up when a value is available. This also adds an update method to cf.Checkpoint.

Using both together to sum values from a stream:

total = cf.Checkpoint("total", default=0)

@cf.task()
def sum_stream(stream: cf.Stream[int]) -> int:
    with cf.suspense(timeout=1.0):
        for value in stream:
            total.update(lambda x: x + value)
    return total.get()