[C++] Simplify creating a scan node

[asfimport]

The approach taken here will depend on the approach taken by ARROW-12311. If ScanOptions is completely hidden then we need a way to create a scan node from a ScannerBuilder and a Dataset. On the other hand, if we get rid of ScannerBuilder and only have ScanOptions then no work needs to be done here.

Reporter: Weston Pace / @westonpace

_{Note: This issue was originally created as ARROW-15261. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.