feat: add GroupBy.__iter__

[tswast]

Note: this is a work in progress. We have two choices for the interface, and I find myself flip flopping between the two:

1. Return an iterable of pandas objects, similar to to_pandas_batches(). To make sure we end up with all the rows together, this would mean (a) create a struct of all non-grouped fields and (b) array_agg those structs and (c) unpack those arrays and structs into DataFrame objects locally.
2. Return an iterable of bigframes objects, each filtered to match rows belonging to the corresponding group. This would involve running a query and iterating through the results to get all the key values and then for each key value, return a DataFrame (or Series) with the corresponding filter attached.

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Fixes internal bug 383638782 🦕

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