arrow

ARROW-13087: [R] Expose Parquet ArrowReaderProperties::coerce_int96_timestamp_unit_

This PR adds a setter and getter for `ArrowReaderProperties::coerce_int96_timestamp_unit_`.

Reprex:

``` r
library(arrow, warn.conflicts = FALSE)

tf <- tempfile()
write_parquet(
  data.frame(a = as.POSIXct("2001-01-01 12:34:56.789", tz = "UTC")),
  tf,
  use_deprecated_int96_timestamps = TRUE, version = "1.0"
)

props <- ParquetArrowReaderProperties$create()
props$set_coerce_int96_timestamp_unit(TimeUnit$MILLI)
props$coerce_int96_timestamp_unit() == TimeUnit$MILLI
#> [1] TRUE

(p <- read_parquet(tf, props = props, as_data_frame = FALSE))
#> Table
#> 1 rows x 1 columns
#> $a <timestamp[ms]>
#>
#> See $metadata for additional Schema metadata
p$a
#> ChunkedArray
#> [
#>   [
#>     2001-01-01 12:34:56.789
#>   ]
#> ]
```

Closes #12070 from paleolimbot/r-int96

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>