[SPARK-14353] Dataset Time Window `window` API for R

## What changes were proposed in this pull request?

The `window` function was added to Dataset with [this PR](https://github.com/apache/spark/pull/12008).
This PR adds the R API for this function.

With this PR, SQL, Java, and Scala will share the same APIs as in users can use:
 - `window(timeColumn, windowDuration)`
 - `window(timeColumn, windowDuration, slideDuration)`
 - `window(timeColumn, windowDuration, slideDuration, startTime)`

In Python and R, users can access all APIs above, but in addition they can do
 - In R:
   `window(timeColumn, windowDuration, startTime=...)`

that is, they can provide the startTime without providing the `slideDuration`. In this case, we will generate tumbling windows.

## How was this patch tested?

Unit tests + manual tests

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #12141 from brkyvz/R-windows.