arrow

ARROW-15016: [R] `show_exec_plan` for an `arrow_dplyr_query` (#13541)

This PR adds `show_exec_plan()` will allow users to inspect the ExecPlan, in a similar way to dplyr's `show_query()`. 
```r
mtcars %>% 
  arrow_table() %>% 
  filter(mpg > 20) %>% 
  mutate(x = gear/carb) %>%
  group_by(cyl) %>% 
  show_exec_plan()
#> ExecPlan with 3 nodes:
#> 2:ProjectNode{projection=[mpg, cyl, disp, hp, drat, wt, qsec, vs, am, gear, carb, "x": divide(cast(gear, {to_type=double, allow_int_overflow=false, allow_time_truncate=false, allow_time_overflow=false, allow_decimal_truncate=false, allow_float_truncate=false, allow_invalid_utf8=false}), cast(carb, {to_type=double, allow_int_overflow=false, allow_time_truncate=false, allow_time_overflow=false, allow_decimal_truncate=false, allow_float_truncate=false, allow_invalid_utf8=false}))]}
#>   1:FilterNode{filter=(mpg > 20)}
#>     0:TableSourceNode{}
```
Some design considerations are discussed in the [design doc](https://docs.google.com/document/d/1Ep8aV4jDsNCCy9uv1bjWY_JF17nzHQogv0EnGJvraQI/edit?usp=sharing).

Summary of the approach:
* I opted for the `show_exec_plan()` name as I believe it aligns well with the purpose of this PR: to expose the ExecPlan (in its raw state).
* We could later extend, beautify the output (e.g. use {cli} and / or `show_query()` / `explain()` methods).

Authored-by: Dragoș Moldovan-Grünfeld <dragos.mold@gmail.com>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>