Use Display instead of ToString in Rust generators (#18633)

samples/client/others/rust/hyper/api-with-ref-param/src/models/color.rs

@@ -23,12 +23,12 @@ pub enum Color {
 
 }
 
-impl ToString for Color {
-    fn to_string(&self) -> String {
+impl std::fmt::Display for Color {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
-            Self::Red => String::from("RED"),
-            Self::Green => String::from("GREEN"),
-            Self::Blue => String::from("BLUE"),
+            Self::Red => write!(f, "RED"),
+            Self::Green => write!(f, "GREEN"),
+            Self::Blue => write!(f, "BLUE"),
         }
     }
 }