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ARROW-9749: [C++][GLib][Python][R][Ruby][Dataset] Introduce FragmentScanOptions, consolidate ScanContext/ScanOptions

- ScanContext/ScanOptions have been merged, since they were essentially always passed together.
- For scan options that are specific to a scan (e.g. CSV conversion options), a new FragmentScanOptions has been added to ScanOptions. Currently only CSV has this and it only wraps csv::ConvertOptions; follow up issues can tackle the rest.
- GLib, Python, R, and Ruby bindings have been updated.

Closes #9686 from lidavidm/arrow-9749

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>