Fix UCL object memory leak in Lua integration

Fix UCL object memory leak in Lua integration

When UCL objects are passed to Lua via ucl_object_push_lua_unwrapped(),
the reference count is incremented but the garbage collector finalizer
was not being called, causing memory leaks.

The issue is that Lua 5.1/LuaJIT does not support __gc metamethod for
tables, only for userdata. This fix adds proper garbage collection
support for both Lua versions:

- For Lua 5.1/LuaJIT: Add __gc metamethod to the userdata stored in
  the table at index [0], which properly triggers reference cleanup
- For Lua 5.2+: Use the existing table __gc metamethod which works
  correctly in newer Lua versions

This ensures that ucl_object_unref() is called exactly once when the
Lua wrapper is garbage collected, preventing memory leaks.