[SPARK-19427][PYTHON][SQL] Support data type string as a returnType argument of UDF

## What changes were proposed in this pull request?

Add support for data type string as a return type argument of `UserDefinedFunction`:

```python
f = udf(lambda x: x, "integer")
 f.returnType

## IntegerType
```

## How was this patch tested?

Existing unit tests, additional unit tests covering new feature.

Author: zero323 <zero323@users.noreply.github.com>

Closes #16769 from zero323/SPARK-19427.