Expose RowAccumulator in physical_plan

[iajoiner]

Which issue does this PR close?

Closes #3138.

Rationale for this change

What changes are included in this PR?

Expose RowAccumulator to the public in physical_plan so that other crates can implement AggregateExpr with that feature.

Are there any user-facing changes?

Expose RowAccumulator to the public in physical_plan.

[alamb]

Thanks @iajoiner !

[alamb]

You may also want to consider a pub use in https://github.com/apache/arrow-datafusion/blob/master/datafusion/core/src/physical_plan/mod.rs so you can use this directly from the datafusion crate (rather than having to use the datafusion-physical-plan crate as well)

[iajoiner]

@alamb Thanks! Actually that's precisely what I did. I think it is appropriate for RowAccumulator to be located where Accumulator is.

[ursabot]

Benchmark runs are scheduled for baseline = 3c0b1c0 and contender = 5778ef2. 5778ef2 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java

[alamb]

@alamb Thanks! Actually that's precisely what I did. I think it is appropriate for RowAccumulator to be located where Accumulator is.

🤦 -- I misread the diff 😢