[C++] Consolidate ScanOptions and ScanContext

[asfimport]

Currently ScanOptions has two distinct responsibilities: it contains the data selector (and eventually projection schema) for the current scan and it serves as the base class for format specific scan options.

In addition, we have ScanContext which holds the memory pool for the current scan.

I think these classes should be rearranged as follows: ScanOptions will be removed and FileScanOptions will be the abstract base class for format specific scan options. ScanContext will be a concrete struct and contain the data selector, projection schema, a vector of FileScanOptions, and any other shared scan state.

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

PRs and other links:

• GitHub Pull Request #5239

_{Note: This issue was originally created as ARROW-6398. Please see the migration documentation for further details.}