arrow

GH-35542 : [R] Implement schema extraction function (#35543)

``` r
library(arrow)
mtcarrow <- arrow_table(mtcars)
schema(mtcarrow)
#> Schema
#> mpg: double
#> cyl: double
#> disp: double
#> hp: double
#> drat: double
#> wt: double
#> qsec: double
#> vs: double
#> am: double
#> gear: double
#> carb: double
#> 
#> See $metadata for additional Schema metadata
```

<sup>Created on 2023-05-11 with [reprex v2.0.2](https://reprex.tidyverse.org)</sup>

* Closes: #35542

Lead-authored-by: Nic Crane <thisisnic@gmail.com>
Co-authored-by: Dewey Dunnington <dewey@dunnington.ca>
Signed-off-by: Nic Crane <thisisnic@gmail.com>