Do not skip symlinks in user_defined directory during SQL user defined functions loading

Why it matters

Previously, symbolic links in the user_defined directory were skipped when loading SQL user-defined functions, preventing users from organizing or sharing function definitions via symlinks. This feature fixes that limitation, allowing symlinked functions to be recognized and loaded properly, increasing flexibility in managing user-defined functions.

How to use it

Place symbolic links to SQL user-defined function files inside the user_defined directory. ClickHouse will automatically load these functions from symlink targets without skipping them.