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ARROW-16776: [R] dplyr::glimpse method for arrow table and datasets (#13563)

See reprex (sans terminal formatting) in [r/tests/testthat/_snaps/dplyr-glimpse.md](https://github.com/apache/arrow/pull/13563/files#diff-e8d50da600908f077796a43b7600c17d34448671c7975bb8c4056a484ac2999e)

Not all queries can be glimpse()d: some would require evaluating the whole query, which may be expensive (and can't be interrupted yet, see ARROW-11841). 

Note that the existing `print()` methods aren't affected by this. There is still the idea that the print methods for Table/RecordBatch should print some data (ARROW-16777 and others), but that should probably be column-oriented instead of row-oriented like glimpse(). 

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