[C++] Expose `DeclarationInfo`

[asfimport]

DeclarationInfo is just a pair of Declaration and Schema, which are public APIs, and so can be made public API itself. This can be part of or a follow-up on #14485, and will allow implementing extension providers, whose API depends on DeclarationInfo, outside of the Arrow repo.

Reporter: Yaron Gvili / @rtpsw
Assignee: Yaron Gvili / @rtpsw

PRs and other links:

• GitHub Pull Request #14765

_{Note: This issue was originally created as ARROW-18402. Please see the migration documentation for further details.}

[asfimport]

Yaron Gvili / @rtpsw:
cc @westonpace

[asfimport]

Weston Pace / @westonpace:
This is a good idea. I'd like to eventually support a DeserializePlan that doesn't create any sink nodes (so that users can add them later) and this would return a DeclarationInfo.

[asfimport]

Weston Pace / @westonpace:
Fixed by PR #14765