make UDFs in datafusion-functions public

[alamb]

Is your feature request related to a problem or challenge?

We have a special wrapper over to_timestamp that has influxdb specific semantics, but falls back to the built in implementation of to_timestamp.

Now that to_timestamp has been extracted from the core, the only way I could find this UDF was by checking by name

   // Find DataFusion's built in to_timestamp implementation
   let fallback = datafusion::functions::datetime::functions()
     .into_iter()
     .find(|f| f.name() == "to_timestamp")
     .expect("to_timestamp function not found"),

Describe the solution you'd like

It would be nice if I could simply get the to_timestamp function directly as I know exactly what implementation I want, but I can't as it is not pub

Describe alternatives you've considered

No response

Additional context

No response