Request: Improve Monotoniciy API

[alamb]

Is your feature request related to a problem or challenge?

While reviewing #9869 from @tinfoil-knight I was confused about the ScalarUDFImpl::monotonicity API

// This function specifies monotonicity behaviors for User defined scalar functions.
fn monotonicity(&self) -> Result<Option<Vec<Option<bool>>>, DataFusionError> {..}

The challenge is the cognative complexity of understanding what Result<Option<Vec<Option<bool>>> means

The actual code is defined in terms FuncMonotonicity https://github.com/apache/arrow-datafusion/blob/57c0bc65280c467ef6f5a498c6e78a616401b286/datafusion/expr/src/signature.rs#L350-L356

However, since it is a typedef it doesn't show up in the Rust docs

Describe the solution you'd like

An easy to understand Monotonicity API

Describe alternatives you've considered

I think it would be great to have an API like this:

// This function specifies monotonicity behaviors for User defined scalar functions.
fn monotonicity(&self) -> Result<Monotonicity>, DataFusionError> {..}

And then we could make Monotonicity an enum that captured the relevant data. This would both be easier to understand and likely more efficient as well

Something like

/// Monotonicity of a function with respect to its arguments.
///
/// A function is [monotonic] if it preserves the relative order of its inputs. 
///
/// [monotonic]: https://en.wikipedia.org/wiki/Monotonic_function
pub enum Monotonicity {
  /// not monotonic or unknown monotonicity
  None,
  /// Increasing with respect to all of its arguments
  Increasing,
  /// Decreasing with respect to all of its arguments
  Decreasing,
  /// Each element of this vector corresponds to an argument and indicates whether
  /// the function's behavior is monotonic, or non-monotonic/unknown for that argument, namely:
  /// - `None` signifies unknown monotonicity or non-monotonicity.
  /// - `Some(true)` indicates that the function is monotonically increasing w.r.t. the argument in question.
  /// - Some(false) indicates that the function is monotonically decreasing w.r.t. the argument in question.
  Mixed(Vec<bool>),
}

I think this would be much eaiser to reason about

Additional context

No response

[tinfoil-knight]

take

[alamb]

#10504 introduces a new API for boundary propagation, so the API challenge described in this ticket is no longer relevant. Thus closing