Some `datafusion-spark` expressions are missing the physical implementation

### Describe the bug

I wanted to add `base64` support in Comet using the expression from `datafusion-spark`, but it has no implementation, so I had to implement a custom version in Comet. This doesn't seem ideal.

```rust
fn invoke_with_args(&self, _args: ScalarFunctionArgs) -> Result {
exec_err!("invoke should not be called on a simplified {} function", self.name())
}
```

Impacted functions:

- [ ] base64
- [ ] unix
- [ ] trunc
- [ ] if
- [ ] date_diff
- [ ] time_trunc
- [ ] date_trunc
- [ ] add_months
- [ ] date_part

### Additional context

https://discord.com/channels/885562378132000778/1413656077014143008/1499408538429030421