## What changes were proposed in this pull request?

This PR adds `udf` decorator syntax as proposed in [SPARK-19160](https://issues.apache.org/jira/browse/SPARK-19160).

This allows users to define UDF using simplified syntax:

```python
from pyspark.sql.decorators import udf

udf(IntegerType())
def add_one(x):
    """Adds one"""
    if x is not None:
        return x + 1
 ```

without need to define a separate function and udf.

## How was this patch tested?

Existing unit tests to ensure backward compatibility and additional unit tests covering new functionality.

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

Closes #16533 from zero323/SPARK-19160.