[C++] Adjust to new format for Substrait optional arguments

[asfimport]

Substrait is presumably going to change how it defines optional arguments in substrait-io/substrait#342 .

This change will require a corresponding change in Acero (this should also bring Acero in line with Ibis & Isthmus).

Reporter: Weston Pace / @westonpace
Assignee: Weston Pace / @westonpace

PRs and other links:

• GitHub Pull Request #14415

_{Note: This issue was originally created as ARROW-17966. Please see the migration documentation for further details.}

[asfimport]

Weston Pace / @westonpace:
Issue resolved by pull request 14415
#14415