[C++] Create Scanner interface

[asfimport]

This will help kick off the work of creating AsyncScanner.  The interface should not include references to the deprecated Scan() or ScanTask.

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

PRs and other links:

• GitHub Pull Request #9947

_{Note: This issue was originally created as ARROW-12288. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Issue resolved by pull request 9947
#9947