## What changes were proposed in this pull request?

Since we could not directly define the array type in R, this PR proposes to support array types in R as string types that are used in `structField` as below:

```R
jsonArr <- "[{\"name\":\"Bob\"}, {\"name\":\"Alice\"}]"
df <- as.DataFrame(list(list("people" = jsonArr)))
collect(select(df, alias(from_json(df$people, "array<struct<name:string>>"), "arrcol")))
```

prints

```R
      arrcol
1 Bob, Alice
```

## How was this patch tested?

Unit tests in `test_sparkSQL.R`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17178 from HyukjinKwon/SPARK-19828.