Add support for factorial function

### Description

Currently factorial falls back due to lack of support in Velox.
We need to add factorial for Spark in Velox

The issue names factorial support in Spark through Velox but does not identify files, tests, or an entry point. Start by locating Spark's factorial handling and the corresponding Velox function support. Done means factorial no longer falls back for Spark and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.