ARROW-18402: [C++] Expose DeclarationInfo

[rtpsw]

See https://issues.apache.org/jira/browse/ARROW-18402

[rtpsw]

cc @westonpace

[github-actions]

https://issues.apache.org/jira/browse/ARROW-18402

[westonpace]

Looks good, thanks! This will be needed for extension relations.

Marking "request changes" because we should also add relation.h to src/arrow/engine/api.h

[westonpace]

Suggested change

	#include "arrow/compute/exec/exec_plan.h"
	#include <memory>
	#include "arrow/compute/exec/exec_plan.h"

Minor IWYU nit

[ursabot]

Benchmark runs are scheduled for baseline = 9b0f3d7 and contender = ce73832. ce73832 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.03% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.52% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] ce738320 ec2-t3-xlarge-us-east-2
[Failed] ce738320 test-mac-arm
[Finished] ce738320 ursa-i9-9960x
[Finished] ce738320 ursa-thinkcentre-m75q
[Finished] 9b0f3d71 ec2-t3-xlarge-us-east-2
[Finished] 9b0f3d71 test-mac-arm
[Finished] 9b0f3d71 ursa-i9-9960x
[Finished] 9b0f3d71 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