ARROW-13238 ensured that asynchronous dataset scans are internally backed by ExecPlans, allowing easier integration with new ExecNode sinks.

However synchronous scans are still backed by FilterAndProjectScanTask and are not consumable using ExecNodes. Ideally, we should route synchronous scans through an ExecPlan as well.

Reporter: Ben Kietzman / @bkietz

Related issues:

• [C++][Dataset] Make async Scanner the default (relates to)
• [C++][Dataset] Simplify ScanOptions after complexity has moved to ScanNode (is depended upon by)

_{Note: This issue was originally created as ARROW-13328. Please see the migration documentation for further details.}