[C++] Create scan node that doesn't rely on the merged generator #20355

See the parent task for more details. This sub-task is to create the initial scan node. It appears that some changes will be needed in each of the format classes. Those changes can be done as follow-on sub-tasks.

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

PRs and other links:

• GitHub Pull Request #13782

_{Note: This issue was originally created as ARROW-17287. Please see the migration documentation for further details.}