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ARROW-15838: [R] Coalesce join keys in full outer join (#14286)

Also:
 * Added support for `keep` argument in join.
 * Cleaned up the C++ code for the join so we directly accept the enum.

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>