Draft: Extend Expr::ScalarUDF to support Expr for ScalarUDF

[2010YOUY01]

Which issue does this PR close?

POC for #8157

Rationale for this change

The rationale is described in #8180
This PR is just used to demonstrate approach 1 in #8180

What changes are included in this PR?

1. When Expr for functions is created using call_fn(), it first stores a ScalarUDF with only name and stub implementation inside
2. During 1st analyzing rule, resolve name-only UDF into the implementation from the function registry

This approach requires less change to existing code.
(I'm wondering do you have any thoughts on this one? @alamb )

Are these changes tested?

Are there any user-facing changes?

[alamb]

If you can make this work, it seems like a good idea to me.

However, it is not clear to me how the subsequent passes will know they have to resolve this particular scalar UDF though 🤔 so I am not sure this approach is viable

Specifically, how would the code look that the analysis pass used to test if a ScalarUDF was unresolved or not?

[2010YOUY01]

That's a great point, without enum the implementation seems a bit vulnerable to bugs
I think we can either make Expr::ScalarFunction or Expr::ScalarUDF a enum, since the long-term goal is to remove BuiltinScalarFunction they should be same.
I'll check which way can make future migration easier

pub enum ScalarFunctionDefinition {
  /// Resolved to a user defined function
  UDF(ScalarUDF),
  /// A scalar function that will be called by name
  Name(Arc<str>),
}

#[derive(Clone, PartialEq, Eq, Hash, Debug)]
pub struct ScalarUDF {
    /// The function
    pub fun: ScalarFunctionDefinition,
    /// List of expressions to feed to the functions as arguments
    pub args: Vec<Expr>,
}

pub enum ScalarFunctionDefinition {
  /// Resolved to a built in scalar function
  /// (will be removed long term)
  BuiltIn(built_in_function::BuiltinScalarFunction),
  /// Resolved to a user defined function
  UDF(ScalarUDF),
  /// A scalar function that will be called by name
  Name(Arc<str>),
}

#[derive(Clone, PartialEq, Eq, Hash, Debug)]
pub struct ScalarFunction {
    /// The function
    pub fun: ScalarFunctionDefinition,
    /// List of expressions to feed to the functions as arguments
    pub args: Vec<Expr>,
}