feat: generalize `sum_expr` in `GroupByExpr` (#36) #19

Job	Run time
lint-and-test / Test Suite	9m 19s
lint-and-test / Check Package	2m 16s
lint-and-test / Clippy	2m 10s
lint-and-test / Format	13s
lint-and-test / Unused Dependencies	6m 56s
Release	2m 15s
	23m 9s

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