Add support for pip install prefect[dask]

[Andrew-S-Rosen]

With the addition of the prefect-dask codebase directly into prefect, I think it's worth considering adding support for pip install prefect[dask] as a convenience option for end-users. I have enabled this by modifying setup.py.

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[chrisguidry]

Thanks @Andrew-S-Rosen, this also aligns with what we'd like to do for some of our other collections, thanks for kicking this off!