GH-47390: [C++][Acero] Allow for any type of scalar in Pivot longer features

[gitmodimo]

Rationale for this change

Allow supplying any scalar as feature

What changes are included in this PR?

Support scalar in PivotLonger

Are these changes tested?

Yes

Are there any user-facing changes?

Yes.

This PR includes breaking changes to public APIs.

PivotLongerRowTemplate::feature_values is now a std::vector<std::shared_ptr<Scalar>> while it used to be std::vector<std::string>).
The PivotLongerRowTemplate still allows passing a vector of strings, though.

• GitHub Issue: [C++][Acero] Add support for any scalar type in pivot longer #47390

[github-actions]

⚠️ GitHub issue #47390 has been automatically assigned in GitHub to PR creator.

[github-actions]

Thank you for your contribution. Unfortunately, this pull request has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this PR will be closed in 14 days. Feel free to re-open this if it has been closed in error. If you do not have repository permissions to reopen the PR, please tag a maintainer.

[gitmodimo]

@zanmato1984 @westonpace @pitrou What do I need to do to get my PRs reviewed? I am waiting over a year with no response. Bot is now trying to remove what I believe valid contribution.

[pitrou]

@zanmato1984 Do you think this is a reasonable addition?

[zanmato1984]

@pitrou Yes, I think the capability itself is reasonable. Feature columns produced by pivot-longer can naturally contain typed values, for example integer week numbers, dates, or timestamps. Forcing them through strings loses type information and requires downstream casts. Keeping the vector<string> constructor also preserves the common existing spelling.

I left one inline comment about handling null scalar pointers safely.

Also, since the public feature_values member type changes, I think the Breaking Change label should be added even though the legacy constructor is retained.

[gitmodimo]

@zanmato1984 good catch. I believe I have no permissions to modify labels. Can you add it?