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 $metadata for additional Schema metadatap$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