Adds PrefectDistributedClient to prefect-dask

[desertaxle]

This PR adds a PrefectDistributedClient to prefect-dask. The client is a drop in replacement for the client from distributed for submitting work to a Dask cluster. This client will power the new task runner, but splitting out this client allows users to take advantage of Prefect features without changing their existing Dask code.

Example

An example flow with tasks submitted to Dask via the PrefectDistributedClient

from prefect.flows import flow
from prefect.tasks import task
from prefect_dask import PrefectDistributedClient

@task
def square(x):
    return x**2


@task
def neg(x):
    return -x


@task
def summation(args):
    return sum(args)


@flow
def my_flow():
    with PrefectDistributedClient() as client:
        square_futures = client.map(square, range(10))
        neg_futures = client.map(neg, square_futures)
        total_future = client.submit(
            summation,
            args=neg_futures,
        )
        return total_future.result()


if __name__ == "__main__":
   my_flow()

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.
• If this pull request adds new functionality, it includes unit tests that cover the changes
• This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.

For documentation changes:

• This pull request includes redirect settings in netlify.toml for files that are removed or renamed.

For new functions or classes in the Python SDK:

• This pull request includes helpful docstrings.
• If a new Python file was added, this pull request contains a stub page in the Python SDK docs and an entry in mkdocs.yml navigation.

[desertaxle]

I had to change this order to ensure the client gets the correct API URL from the current settings.

[cicdw]

LGTM!