[C++] Simplify creating a scan node

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](https://issues.apache.org/jira/browse/ARROW-15261) / @westonpace

_{**Note**: *This issue was originally created as [ARROW-15261](https://issues.apache.org/jira/browse/ARROW-15261). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by reading ARROW-12311 and the migration documentation linked in this issue, since the implementation depends on whether ScanOptions or ScannerBuilder remains public. Then inspect the existing scan-node creation path involving ScannerBuilder and Dataset. Done means the chosen API can create a scan node consistently under the outcome of ARROW-12311.

Written by the indexing model from the issue text.