`var(distinct)` support

[WinkerDu]

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[Jefffrey]

Checklist of things needed to be implemented. For reference existing implementation:

https://github.com/apache/datafusion/blob/2a08013af3ccf703bee202c959b40bb0d35bdea1/datafusion/functions-aggregate/src/variance.rs

Distinct accumulator for variance, see example for median:

https://github.com/apache/datafusion/blob/2a08013af3ccf703bee202c959b40bb0d35bdea1/datafusion/functions-aggregate/src/median.rs#L511-L542

Make sure to wire distinct accumulator into the UDAF:

https://github.com/apache/datafusion/blob/2a08013af3ccf703bee202c959b40bb0d35bdea1/datafusion/functions-aggregate/src/median.rs#L151-L184

https://github.com/apache/datafusion/blob/2a08013af3ccf703bee202c959b40bb0d35bdea1/datafusion/functions-aggregate/src/median.rs#L132-L149

Add tests here:

https://github.com/apache/datafusion/blob/2a08013af3ccf703bee202c959b40bb0d35bdea1/datafusion/sqllogictest/test_files/aggregate.slt