Support custom window frame with AVG aggregate function

[jonmmease]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I would like DataFusion to support using the avg aggregate function in a window with custom frame.

Here is a working example using sum:

❯ select sum(column1) OVER(ROWS BETWEEN 1 PRECEDING AND CURRENT ROW) from (values (1.0), (2.0), (3.0)) as sq;
+-----------------+
| SUM(sq.column1) |
+-----------------+
| 1               |
| 3               |
| 5               |
+-----------------+

But it does not work for avg:

❯ select avg(column1) OVER(ROWS BETWEEN 1 PRECEDING AND CURRENT ROW) from (values (1.0), (2.0), (3.0)) as sq;
ArrowError(ExternalError(ArrowError(ExternalError(NotImplemented("Retractable Accumulator hasn't been implemented for Avg { name: \"AVG(sq.column1)\", expr: Column { name: \"column1\", index: 0 }, data_type: Float64 } yet")))))

Describe the solution you'd like
I would like avg to support use in windows with custom frames.

Additional context
Looking at the implementation briefly, I think AvgAccumulator needs an implementation of retract_batch, similar to this implementation for SumAccumulator:

https://github.com/apache/arrow-datafusion/blob/3cc607de4ce6e9e1fd537091e471858c62f58653/datafusion/physical-expr/src/aggregate/sum.rs#L256-L262