ARROW-16700: [C++][R][Datasets] aggregates on partitioning columns

[jvanstraten]

This updates the Scanner node such that it will use the guarantee expression to fill out columns missing from the dataset but guaranteed to be some constant with appropriate scalars, rather than just inserting a null placeholder column. In case both are available, the dataset constructor prefers using the scalar from the guarantee expression over the actual data, since the latter would probably be an array that unnecessarily repeats the constant value.

This is the other part of what was uncovered while analyzing ARROW-16700, the more direct cause being a duplicate of ARROW-16904 (see also #13509 for my fix for that).

[github-actions]

https://issues.apache.org/jira/browse/ARROW-16700

[westonpace]

This looks good. One suggestion for the test.

[drin]

rebased branch

[drin]

I confirmed that the C++ unit tests validate the fix provided in this PR. Still working on an R test that correctly validates the fix.

arrow/cpp/src/arrow/dataset/scanner.cc

Lines 911 to 916 in 6592ad2

	auto guarantee = partial.fragment.value->partition_expression();
	ARROW_ASSIGN_OR_RAISE(
	util::optional<compute::ExecBatch> batch,
	compute::MakeExecBatch(*scan_options->dataset_schema,
	partial.record_batch.value, guarantee));

[westonpace]

I'm sorry but I believe I have sent you on a wild goose chase. I had suggested an R test but, upon re-reading the JIRA, realize that I had forgotten this is not reproducible from R (because it always inserts a project node prior to an aggregate node) and so no R test is needed.

[westonpace]

A few minor suggestions on the unit test and I think this is good to go.

[westonpace]

Thanks both of you for taking care of this.

[ursabot]

Benchmark runs are scheduled for baseline = b3ce0fa and contender = e0ccfa1. e0ccfa1 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Failed] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.59% ⬆️0.0%] test-mac-arm
[Failed ⬇️0.63% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.29% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Failed] e0ccfa11 ec2-t3-xlarge-us-east-2
[Failed] e0ccfa11 test-mac-arm
[Failed] e0ccfa11 ursa-i9-9960x
[Finished] e0ccfa11 ursa-thinkcentre-m75q
[Failed] b3ce0fa7 ec2-t3-xlarge-us-east-2
[Failed] b3ce0fa7 test-mac-arm
[Failed] b3ce0fa7 ursa-i9-9960x
[Finished] b3ce0fa7 ursa-thinkcentre-m75q
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