[C++] Add support for Substrait's RelCommon::Emit

[asfimport]

Any relation in Substrait is allowed to "emit" the output which means reordering the columns (and possibly dropping columns?).

The C++ Substrait consumer currently does not handle this.

This functionality could probably be handled by a project operation in the Arrow execution engine.

Reporter: Weston Pace / @westonpace
Assignee: Vibhatha Lakmal Abeykoon / @vibhatha

Related issues:

• [C++] Infer project nodes lazily to avoid unnecessary chains when consuming Substrait (relates to)

PRs and other links:

• GitHub Pull Request #13914

_{Note: This issue was originally created as ARROW-15584. Please see the migration documentation for further details.}

[asfimport]

Jeroen van Straten / @jvanstraten:
While this could be implemented before ARROW-16986, the way it would be implemented depends heavily on whether that's been done yet, so doing this now might duplicate some work. However, ARROW-16986 probably has lower priority.

[asfimport]

Weston Pace / @westonpace:
Issue resolved by pull request 13914
#13914