[SPARK-20431][SS][FOLLOWUP] Specify a schema by using a DDL-formatted string in DataStreamReader

## What changes were proposed in this pull request?

This pr supported a DDL-formatted string in `DataStreamReader.schema`.
This fix could make users easily define a schema without importing the type classes.

For example,

```scala
scala> spark.readStream.schema("col0 INT, col1 DOUBLE").load("/tmp/abc").printSchema()
root
 |-- col0: integer (nullable = true)
 |-- col1: double (nullable = true)
```

## How was this patch tested?

Added tests in `DataStreamReaderWriterSuite`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18373 from HyukjinKwon/SPARK-20431.