Add ELT support in SQL & Table API

Implement the elt function to extract the n-th input value from a list of inputs.

Description:
The elt function in the ETL pipeline extracts the value at the n-th position from a list of input values. It is similar to array indexing, where the first element is at position 1. This function provides a convenient way to retrieve specific elements from a list of inputs.

Syntax:

elt[n: int, *inputs: str] -> str or None

Arguments:

n: The index of the input value to extract. It should be a positive integer.
*inputs: Variable-length arguments representing the list of inputs.

Returns:
The value at the n-th position in the list of inputs. If the index exceeds the length of the array, the function returns NULL. 

Examples:

Retrieving the second element from a list of strings:

elt(2, 'scala', 'java')
Output: 'java'

Retrieving the second element from a list of mixed types:

result = elt(2, 'a', 1)
Output: 1

See also:

spark:https://spark.apache.org/docs/latest/api/sql/index.html#elt