[SPARK-12903][SPARKR] Add covar_samp and covar_pop for SparkR

Add ```covar_samp``` and ```covar_pop``` for SparkR.
Should we also provide ```cov``` alias for ```covar_samp```? There is ```cov``` implementation at stats.R which masks ```stats::cov``` already, but may bring to breaking API change.

cc sun-rui felixcheung shivaram

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #10829 from yanboliang/spark-12903.