arrow

ARROW-17463: [R] Avoid unnecessary projections (#13954)

Before:

```
> 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{}
```

After:

```
ExecPlan with 5 nodes:
4:SinkNode{}
  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{}
```

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