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

[paleolimbot]

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

Reprex:

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
#>   ]
#> ]

[github-actions]

https://issues.apache.org/jira/browse/ARROW-13087

[github-actions]

⚠️ Ticket has not been started in JIRA, please click 'Start Progress'.

[thisisnic]

All looks good!

[kszucs]

Thanks @paleolimbot and @thisisnic!

[ursabot]

Benchmark runs are scheduled for baseline = 17bf54a and contender = 94d4719. 94d4719 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.0% ⬆️2.14%] ursa-i9-9960x
[Finished ⬇️0.17% ⬆️0.0%] ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python. Runs only benchmarks with cloud = True
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java