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ARROW-16700: [C++][R][Datasets] aggregates on partitioning columns (#13518)

This updates the Scanner node such that it will use the guarantee expression to fill out columns missing from the dataset but guaranteed to be some constant with appropriate scalars, rather than just inserting a null placeholder column. In case both are available, the dataset constructor prefers using the scalar from the guarantee expression over the actual data, since the latter would probably be an array that unnecessarily repeats the constant value.

This is the other part of what was uncovered while analyzing ARROW-16700, the more direct cause being a duplicate of ARROW-16904 (see also https://github.com/apache/arrow/pull/13509 for my fix for that).

Lead-authored-by: Jeroen van Straten <jeroen.van.straten@gmail.com>
Co-authored-by: Aldrin M <octalene.dev@pm.me>
Co-authored-by: octalene <octalene.dev@pm.me>
Co-authored-by: Aldrin Montana <octalene.dev@pm.me>
Co-authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>