rpad expression panics if length input is not a literal value

### Describe the bug

The rpad expression panics if the `length` input is not a literal value.

For example, `rpad(string_column, 10)` works, but `rpad(string_column, int_column)` panics.

This is probably not a common usage, but we should fall back to Spark rather than panic.

### Steps to reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_