avg(distinct) support #2408

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

DataFusion does not avg(distinct ..):

> select avg(distinct a), avg(distinct b) from values(1, 2) t(a, b);
Execution error: avg(DISTINCT) aggregations are not available

Somewhat confusingly DataFusion does appear to support a single distinct avg

> select avg(distinct a) from values(1, 2) t(a, b);
+-------------------+
| avg(DISTINCT t.a) |
+-------------------+
| 1.0               |
+-------------------+
1 row(s) fetched.
Elapsed 0.015 seconds.

But it does this by rewriting effectively rewriting the query select avg(distinct a) to select avg(a) from (select distinct a ...) in the SingleDistinctToGroupBy rule

Describe the solution you'd like
Support multile distinct avg aggregates:

Describe alternatives you've considered
Basically we could follow the model of COUNT(distinct ...) aggregates:

In this case, there is a separate accumulator type that is implemented for count distinct

https://github.com/apache/datafusion/blob/5d08325165c1a7b32e5e35164919e83d46735e98/datafusion/functions-aggregate/src/count.rs#L234-L233

Additional context
Add any other context or screenshots about the feature request here.