fix: Box WindowFunction in ExprFuncKind enum to reduce enum total size

As suggested by `clippy`:

```
warning: large size difference between variants
   --> datafusion/expr/src/expr_fn.rs:772:1
    |
772 | / pub enum ExprFuncKind {
773 | |     Aggregate(AggregateFunction),
    | |     ---------------------------- the second-largest variant contains at least 72 bytes
774 | |     Window(WindowFunction),
    | |     ---------------------- the largest variant contains at least 288 bytes
775 | | }
    | |_^ the entire enum is at least 288 bytes
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant
    = note: `#[warn(clippy::large_enum_variant)]` on by default
help: consider boxing the large fields to reduce the total size of the enum
    |
774 -     Window(WindowFunction),
774 +     Window(Box<WindowFunction>),
    |
```

Author: geoffreyclaude

datafusion/expr/src/expr_fn.rs

@@ -770,7 +770,7 @@ pub trait ExprFunctionExt {
 #[derive(Debug, Clone)]
 pub enum ExprFuncKind {
     Aggregate(AggregateFunction),
-    Window(WindowFunction),
+    Window(Box<WindowFunction>),
 }
 
 /// Implementation of [`ExprFunctionExt`].
@@ -841,7 +841,7 @@ impl ExprFuncBuilder {
                 udwf.params.filter = filter.map(Box::new);
                 udwf.params.null_treatment = null_treatment;
                 udwf.params.distinct = distinct;
-                Expr::WindowFunction(Box::new(udwf))
+                Expr::WindowFunction(udwf)
             }
         };
 
@@ -895,7 +895,7 @@ impl ExprFunctionExt for Expr {
                 ExprFuncBuilder::new(Some(ExprFuncKind::Aggregate(udaf)))
             }
             Expr::WindowFunction(udwf) => {
-                ExprFuncBuilder::new(Some(ExprFuncKind::Window(*udwf)))
+                ExprFuncBuilder::new(Some(ExprFuncKind::Window(udwf)))
             }
             _ => ExprFuncBuilder::new(None),
         };
@@ -935,7 +935,7 @@ impl ExprFunctionExt for Expr {
                 ExprFuncBuilder::new(Some(ExprFuncKind::Aggregate(udaf)))
             }
             Expr::WindowFunction(udwf) => {
-                ExprFuncBuilder::new(Some(ExprFuncKind::Window(*udwf)))
+                ExprFuncBuilder::new(Some(ExprFuncKind::Window(udwf)))
             }
             _ => ExprFuncBuilder::new(None),
         };
@@ -948,7 +948,7 @@ impl ExprFunctionExt for Expr {
     fn partition_by(self, partition_by: Vec<Expr>) -> ExprFuncBuilder {
         match self {
             Expr::WindowFunction(udwf) => {
-                let mut builder = ExprFuncBuilder::new(Some(ExprFuncKind::Window(*udwf)));
+                let mut builder = ExprFuncBuilder::new(Some(ExprFuncKind::Window(udwf)));
                 builder.partition_by = Some(partition_by);
                 builder
             }
@@ -959,7 +959,7 @@ impl ExprFunctionExt for Expr {
     fn window_frame(self, window_frame: WindowFrame) -> ExprFuncBuilder {
         match self {
             Expr::WindowFunction(udwf) => {
-                let mut builder = ExprFuncBuilder::new(Some(ExprFuncKind::Window(*udwf)));
+                let mut builder = ExprFuncBuilder::new(Some(ExprFuncKind::Window(udwf)));
                 builder.window_frame = Some(window_frame);
                 builder
             }