[C++] Create basic AsyncScanner implementation #28095
Description
This will also add Async default implementations to the fragments & formats. These will initially just defer to the synchronous implementation via the serial executor which will lead to rotten performance but we can add proper async implementations in the follow-up tasks.
Reporter: Weston Pace / @westonpace
Assignee: Weston Pace / @westonpace
PRs and other links:
Note: This issue was originally created as ARROW-12289. Please see the migration documentation for further details.