Deprecate Expr for functions that take only specific literals

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

This is the original comment that leads to this issue: https://github.com/apache/datafusion-python/pull/1484#discussion_r3079835791

**Describe the solution you'd like**

Add deprecation warnings, then remove`Expr` type hint in a later release

**Describe alternatives you've considered**

Leave as is

**Additional context**

We only allow specific values, so we can check them at the function invocation.