[SPARK-22844][R] Adds date_trunc in R API

## What changes were proposed in this pull request?

This PR adds `date_trunc` in R API as below:

```r
> df <- createDataFrame(list(list(a = as.POSIXlt("2012-12-13 12:34:00"))))
> head(select(df, date_trunc("hour", df$a)))
  date_trunc(hour, a)
1 2012-12-13 12:00:00
```

## How was this patch tested?

Unit tests added in `R/pkg/tests/fulltests/test_sparkSQL.R`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #20031 from HyukjinKwon/r-datetrunc.