Introduce sum_distinct() function to dataframe

[WinkerDu]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
see #2405 , a dataframe interace for sum(distinct) usage

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[alamb]

Perhaps a function like sum_distinct (following the pattern of count_distinct) would be good

https://github.com/apache/arrow-datafusion/blob/c3c02cf5e881c9c5020ad87417716dd932e69a69/datafusion/core/src/logical_plan/mod.rs#L43