[Experimental] Add cardinality argument to GroupBy.aggregate

[rjzamora]

Adds an optional cardinality argument to GroupBy.aggregate:

"""
cardinality : float or "infer", optional
    Approximate ratio of aggregated data size with respect to the
    initial data size. If specified, this ratio will be used to override
    the defaults for ``split_every``, ``split_out``, and ``shuffle``.
    If ``"infer"`` is specified, the first non-empty partition will be
    used to estimate the global cardinality ratio.
"""

This PR is related to the discussion in #9406 (on setting good defaults automatically). Note that the specific heuristics used to set split_every and split_out in this PR may need to be tweaked - More benchmarking is necessary.

• Tests added / passed
• Passes pre-commit run --all-files

[ian-r-rose]

I think it's important to allow split_every to be one (cf #9406 (comment)), so that there isn't any repartitioning to fewer partitions before shuffling. I found this makes a significant difference for the case when there are fewer, larger partitions.

Otherwise, this looks like a reasonable heuristic to me.

[rjzamora]

Closing this for now.