Minor: Move Monotonicity to expr crate

[2010YOUY01]

Which issue does this PR close?

NA

Rationale for this change

I'm working on something related to functions and would like to use Monotonicity inside expr crate.
But now Monotonicity is defined in physical-expr crate, which depends on expr
Since it should be part of the function definition, it's better to move it into expr crate.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[alamb]

Thank you @2010YOUY01 -- this looks like a good idea to me. I think we should avoid a breaking API change, either through pub use or (better in my opinion) a deprecated function

[alamb]

Perhaps we can leave a pub use datafusion_expr::FuncMonotonicty in this module to ease backwards compatibility?

[alamb]

Can we please avoid a breaking api change by at least adding pub use datafusion_expr::get_func_monotonicity in this file?

However, since we are going to move this code anyways, what would you think about putting this code as a method on BuiltinScalarFunction itself, so that it was easier to find

Something like

impl BuiltinScalarFunction {
  pub fn monotonicity(&self) -> Option<FuncMonotonicity> {
  ...
   }
}

And leaving the get_func_monotonicity as a deprecated function, perhaps like

#[deprecated(message="use BuiltinScalarFunction::monotonicity")]
pub fn get_func_monotonicity(fun: &BuiltinScalarFunction) -> Option<FuncMonotonicity> {
  fun.monotonicity()
}

[2010YOUY01]

Thank you, I also think deprecation is better

Clicked wrong button -- please avoid breaking API change

[alamb]

Thank you @2010YOUY01