[SPARK-24054][R] Add array_position function / element_at functions
## What changes were proposed in this pull request?
This PR proposes to add array_position and element_at in R side too.
array_position:
```r
df <- createDataFrame(cbind(model = rownames(mtcars), mtcars))
mutated <- mutate(df, v1 = create_array(df$gear, df$am, df$carb))
head(select(mutated, array_position(mutated$v1, 1)))
```
```
array_position(v1, 1.0)
1 2
2 2
3 2
4 3
5 0
6 3
```
element_at:
```r
df <- createDataFrame(cbind(model = rownames(mtcars), mtcars))
mutated <- mutate(df, v1 = create_array(df$mpg, df$cyl, df$hp))
head(select(mutated, element_at(mutated$v1, 1)))
```
```
element_at(v1, 1.0)
1 21.0
2 21.0
3 22.8
4 21.4
5 18.7
6 18.1
```
```r
df <- createDataFrame(cbind(model = rownames(mtcars), mtcars))
mutated <- mutate(df, v1 = create_map(df$model, df$cyl))
head(select(mutated, element_at(mutated$v1, "Valiant")))
```
```
element_at(v3, Valiant)
1 NA
2 NA
3 NA
4 NA
5 NA
6 6
```
## How was this patch tested?
Unit tests were added in `R/pkg/tests/fulltests/test_sparkSQL.R` and manually tested. Documentation was manually built and verified.
Author: hyukjinkwon <gurwls223@apache.org>
Closes #21130 from HyukjinKwon/sparkr_array_position_element_at.
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