Support array_agg as a sliding window aggregate by implementing retract_batch

[FjodorGit]

Is your feature request related to a problem or challenge?

array_agg currently works as a window aggregate for ever-expanding frames such as ROW BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, but it cannot be used with bounded/sliding frames because its accumulator does not implement retract_batch.

Describe the solution you'd like

It would be useful to implement retract_batch for at least the non-DISTINCT array_agg accumulator such that at least such a query would be possible

SELECT array_agg(x) OVER (
	ORDER BY t 
	ROWS BETWEEN 1 PRECEDING AND CURRENT ROW
)
FROM ...