`approx_distinct` should be leveraging bitmap for counting u8/16 and i8/16

[jimexist]

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approx_distinct should be leveraging bitmap for counting u8/16 and i8/16

Describe the solution you'd like

Using a bitmap is more efficient

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[alamb]

See code at https://github.com/apache/arrow-datafusion/blob/397110ab6948ea80a14155d65acaf55e23fd624e/datafusion/src/physical_plan/expressions/approx_distinct.rs#L88-L91

[Weijun-H]

Why don't we also support u32/i32 🤔 ?

[alamb]

there is no reason I know of