### What changes were proposed in this pull request?
This PR makes `element_at` in PySpark able to take PySpark `Column` instances.
### Why are the changes needed?
To match with Scala side. Seems it was intended but not working correctly as a bug.
### Does this PR introduce any user-facing change?
Yes. See below:
```python
from pyspark.sql import functions as F
x = spark.createDataFrame([([1,2,3],1),([4,5,6],2),([7,8,9],3)],['list','num'])
x.withColumn('aa',F.element_at('list',x.num.cast('int'))).show()
```
Before:
```
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/.../spark/python/pyspark/sql/functions.py", line 2059, in element_at
return Column(sc._jvm.functions.element_at(_to_java_column(col), extraction))
File "/.../spark/python/lib/py4j-0.10.8.1-src.zip/py4j/java_gateway.py", line 1277, in __call__
File "/.../spark/python/lib/py4j-0.10.8.1-src.zip/py4j/java_gateway.py", line 1241, in _build_args
File "/.../spark/python/lib/py4j-0.10.8.1-src.zip/py4j/java_gateway.py", line 1228, in _get_args
File "/.../forked/spark/python/lib/py4j-0.10.8.1-src.zip/py4j/java_collections.py", line 500, in convert
File "/.../spark/python/pyspark/sql/column.py", line 344, in __iter__
raise TypeError("Column is not iterable")
TypeError: Column is not iterable
```
After:
```
+---------+---+---+
| list|num| aa|
+---------+---+---+
|[1, 2, 3]| 1| 1|
|[4, 5, 6]| 2| 5|
|[7, 8, 9]| 3| 9|
+---------+---+---+
```
### How was this patch tested?
Manually tested against literal, Python native types, and PySpark column.
Closes#25950 from HyukjinKwon/SPARK-29240.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
fda0e6e48d