[SQL] Add CASE WHEN equivalent for the DECODE function

[art000109]

Currently, the decode function is used only for decoding strings. However, a CASE WHEN equivalent has been available in the Spark SQL API since version 3.2.0

https://spark.apache.org/docs/latest/api/sql/string-functions/#decode

> SELECT decode(encode('abc', 'utf-8'), 'utf-8');
 abc
> SELECT decode(6, 1, 'Southlake', 2, 'San Francisco', 3, 'New Jersey', 4, 'Seattle', 'Non domestic');
 Non domestic

[ybapat]

I've opened a PR to expose the multi-argument Oracle-style decode in PySpark: #57568

The Scala implementation already handles the CASE WHEN form via object Decode { def createExpr(...) }, but PySpark's decode() only forwarded the 2-arg binary form. The PR updates both the classic and Connect paths to dispatch based on argument count.