Implement filtering with flat masks

[hombit]

Feature request

It would be great if we can filter a nested series with a "flat" mask, boolean array of the same size as the flatten series.

filtered = nested_series[nested_series.nest["a"] + nested_series.nest["b"] > 10.0]

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[hombit]

Unfortunately it looks like it is impossible to implement on the extension array level: pd.Seres.__getitem__ checks if the input boolean indexer has the same number of elements as the series and doesn't call extension array's __getitem__ if the shape mis-matches.