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ARROW-13344: [R] Initial bindings for ExecPlan/ExecNode

This PR adds support for both scalar and group-by aggregation via dplyr::summarize(). Only the functions sum, any, and all are wired up. Followup issues (both bugs and features):

* [C++] Aggregation nodes seem not to respect FunctionOptions, or else I'm not passing them in correctly (ARROW-13497)
* [C++] ScanNode takes filter but doesn't filter (ARROW-13498)
* [R] Aggregation on expression doesn't NSE correctly (ARROW-13499)
* [R] Bindings for mean, var, sd aggregation (ARROW-13528)
* [R] Bindings for count aggregation (ARROW-13501)
* [R] Bindings for min/max aggregation (ARROW-13502)
* [R] Handle summarize() with 0 arguments or no aggregate functions (ARROW-13543)
* [R] Support .groups argument to summarize() (ARROW-13550)
* [C++] MakeScalarAggregateNode and MakeGroupByNode have quite different function signatures, which makes working with the API confusing; GroupBy doesn't let you specify the names of the output columns (ARROW-13482)
* [C++] Grouped aggregation functions all have to be invoked with a `hash_` prefix to the name, which seems unnecessary because you can't call a non-hash-aggregation function in GroupBy and you can't call a hash_ function in ScalarAggregate (ARROW-13451)

Closes #10722 from nealrichardson/scalar-aggregate-node

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