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ARROW-8002: [C++][Dataset][R] Support partitioned dataset writing

Closes #7869 from bkietz/8002-partitioned-write

Lead-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>