Generic function to apply any scalar function on an MV column

Currently, scalar functions supports only Single valued column. Need generic function to apply any scalar function on an MV column

Start by tracing Pinot's scalar-function handling and its support for multi-valued columns, since the issue does not name files, tests, or an entry point. Clarify which scalar functions and multi-valued column types are in scope, then define completion as applying any supported scalar function to an MV column with coverage for the intended cases.

Written by the indexing model from the issue text.