[C++][Python] Optimize aggregate functions to work with batches.

[asfimport]

Most compute functions work with the dataset api and don't load columns. But aggregate functions which are associative could also work: min, max, any, all, sum, product. Even unique and value_counts.

A couple of implementation ideas:

• expand the dataset api to support expressions which return scalars
• add a BatchedArray type which is like a ChunkedArray but with lazy loading

Reporter: A. Coady / @coady

_{Note: This issue was originally created as ARROW-18433. Please see the migration documentation for further details.}

[thisisnic]

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