[SPARK-21602][R] Add map_keys and map_values functions to R

## What changes were proposed in this pull request?

This PR adds `map_values` and `map_keys` to R API.

```r
> df <- createDataFrame(cbind(model = rownames(mtcars), mtcars))
> tmp <- mutate(df, v = create_map(df$model, df$cyl))
> head(select(tmp, map_keys(tmp$v)))
```
```
        map_keys(v)
1         Mazda RX4
2     Mazda RX4 Wag
3        Datsun 710
4    Hornet 4 Drive
5 Hornet Sportabout
6           Valiant
```
```r
> head(select(tmp, map_values(tmp$v)))
```
```
  map_values(v)
1             6
2             6
3             4
4             6
5             8
6             6
```

## How was this patch tested?

Manual tests and unit tests in `R/pkg/tests/fulltests/test_sparkSQL.R`

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18809 from HyukjinKwon/map-keys-values-r.