Add struct accessor to dask-cudf

[NV-jpt]

This PR implements 'Struct Accessor' requested feature in dask-cudf (Issue #8658)

StructMethod class implemented to expose 'field(key)' method in dask-cudf

    Examples
    --------
    >>> s = cudf.Series([{'a': 1, 'b': 2}, {'a': 3, 'b': 4}])
    >>> ds = dask_cudf.from_cudf(s, 2)
    >>> ds.struct.field(0).compute()
    0    1
    1    3
    dtype: int64
    >>> ds.struct.field('a').compute()
    0    1
    1    3
    dtype: int64

[rjzamora]

Thanks for this @NV-jpt - Looking good so far! It looks like you need to resolve some conflicts with branch-21.10 and add pre-commit to your environment to help with formatting.

[rjzamora]

@sarahyurick - I suspect that the most-natural place for the dask-cudf explode feature (requested in #8660) is the struct accessor being added here. It looks like #8729 added the cudf-level feature within the cudf.Series-struct accessor, and not for Series.explode (is this correct @shwina ?)

[rjzamora]

This is getting very close - Nice work @NV-jpt !

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[rjzamora]

This LGTM. My only suggestion is to avoid repeating the same "data" parameters in test_accessor.py (see my comment). Also, could you make the title of the PR slightly more descriptive? For example: "Add struct accessor to dask-cudf"

[NV-jpt]

rerun tests

[shwina]

rerun tests

[beckernick]

rerun tests

[shwina]

rerun tests

[shwina]

Nice work!

[shwina]

@gpucibot merge