[R] Avoid unnecessary projections

[asfimport]

In ExecPlan$Build(), we call Project in a few places, and there is code to make sure that there is at least one ProjectNode in the query in order to remove augmented fields from a Dataset scan (unless the user has added them). As a result, it is possible to get multiple ProjectNodes in a row that are essentially no-op. One example is with grouped aggregation: there is a projection to get the order of the columns back to what R expects, and then a no-op projection after that:

> mtcars |> arrow_table() |> count(cyl) |> explain()
ExecPlan with 6 nodes:
5:SinkNode{}
  4:ProjectNode{projection=[cyl, n]}
    3:ProjectNode{projection=[cyl, n]}
      2:GroupByNode{keys=["cyl"], aggregates=[
      	hash_sum(n, {skip_nulls=true, min_count=1}),
      ]}
        1:ProjectNode{projection=["n": 1, cyl]}
          0:TableSourceNode{}

IDK how significant of a performance impact this would have, but it certainly looks wasteful and should be avoidable.

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #13954

_{Note: This issue was originally created as ARROW-17463. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 13954
#13954