[SPARK-24054][R] Add array_position function / element_at functions

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to add array_position and element_at in R side too.

array_position:

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:

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

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.

[HyukjinKwon]

cc @felixcheung, can you take a look please when you are available?

[SparkQA]

Test build #89729 has finished for PR 21130 at commit 443034a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #89731 has finished for PR 21130 at commit 7f75f5e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[felixcheung]

LGTM!

[viirya]

typo: to to

[viirya]

LGTM except for a tiny typo.

[SparkQA]

Test build #89765 has finished for PR 21130 at commit d1d1bb4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Merged to master.

[HyukjinKwon]

Thank you for reviewing this @felixcheung and @viirya.