Macro based scalar function definitions make IDE harder to trace function usage

### Is your feature request related to a problem or challenge?

I'm revamping the planner in Comet for some recent changes happened in DataFusion, especially for scalar functions.

I found that a few built-in scalar functions are moved into module `expr_fn` with the approach with some macros.

One trouble with this approach is it is hard to trace how does these functions are used across DataFusion. At least in RustRover IDE, it cannot trace the function usage on these function defined by `export_functions` macro.

I'm not sure how do others deal with it?

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by reading the built-in scalar function definitions in the expr_fn module and the export_functions macro usage described in the issue. Investigate how RustRover traces these functions and determine a concrete approach; the work is done when function usages across DataFusion can be traced reliably in the IDE.

Written by the indexing model from the issue text.