Make RowAccumulator public

[iajoiner]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I'd like to implement AggregateExpr wrappers with side effects and it would be great if RowAccumulator is available outside the crate so that my wrapped AggregateExprs can mirror the raw ones in Datafusion-related behavior.

Describe the solution you'd like
Make RowAccumulator public.

Describe alternatives you've considered
Currently I have to turn RowAccumulator off.

Additional context
None