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ARROW-14025: [R][C++] PreBuffer is not enabled when scanning parquet via exec nodes

Also includes an unrelated CSV dataset test, just to confirm that FragmentScanOptions are passed through correctly.

Closes #11386 from nealrichardson/r-prebuffer

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>