## What changes were proposed in this pull request?
This PR proposes to support a DDL-formetted string as schema as below:
```r
mockLines <- c("{\"name\":\"Michael\"}",
"{\"name\":\"Andy\", \"age\":30}",
"{\"name\":\"Justin\", \"age\":19}")
jsonPath <- tempfile(pattern = "sparkr-test", fileext = ".tmp")
writeLines(mockLines, jsonPath)
df <- read.df(jsonPath, "json", "name STRING, age DOUBLE")
collect(df)
```
## How was this patch tested?
Tests added in `test_streaming.R` and `test_sparkSQL.R` and manual tests.
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#18431 from HyukjinKwon/r-ddl-schema.
db44f5f3e8