arrow

ARROW-15875: [R] Expose ReadMetadata for input streams

This PR exposes `RandomAccessFile::GetFileInfo()`, which is particularly useful for S3 files which carry some extra information that is useful to know without reading the whole file.

``` r
# remotes::install_github("paleolimbot/arrow/r@r-file-metadata")
library(arrow, warn.conflicts = FALSE)

bucket <- s3_bucket("ursa-labs-taxi-data")
file <- bucket$OpenInputFile("2019/06/data.parquet")
file$ReadMetadata()
#> $`Content-Length`
#> [1] "120790979"
#>
#> $`Content-Type`
#> [1] "application/x-www-form-urlencoded; charset=utf-8"
#>
#> $ETag
#> [1] "\"f1efd5d76cb82861e1542117bfa52b90-8\""
#>
#> $`Last-Modified`
#> [1] "2020-01-17T16:26:28Z"
```

Closes #12623 from paleolimbot/r-file-metadata

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>