[SPARK-12922][SPARKR][WIP] Implement gapply() on DataFrame in SparkR

## What changes were proposed in this pull request?

gapply() applies an R function on groups grouped by one or more columns of a DataFrame, and returns a DataFrame. It is like GroupedDataSet.flatMapGroups() in the Dataset API.

Please, let me know what do you think and if you have any ideas to improve it.

Thank you!

## How was this patch tested?
Unit tests.
1. Primitive test with different column types
2. Add a boolean column
3. Compute average by a group

Author: Narine Kokhlikyan <narine.kokhlikyan@gmail.com>
Author: NarineK <narine.kokhlikyan@us.ibm.com>

Closes #12836 from NarineK/gapply2.