Add private field to SchemaSettings to prevent struct initialization.

This means future additions to SchemaSettings will not be breaking changes.

schemars/examples/custom_settings.rs

@@ -8,19 +8,16 @@ pub struct MyStruct {
 }
 
 #[derive(JsonSchema)]
-pub enum MyEnum {    
+pub enum MyEnum {
     StringNewType(String),
-    StructVariant {
-        floats: Vec<f32>,
-    }
+    StructVariant { floats: Vec<f32> },
 }
 
 fn main() {
-    let settings = SchemaSettings {
-        option_nullable: true,
-        option_add_null_type: false,
-        ..SchemaSettings::draft07()
-    };
+    let settings = SchemaSettings::draft07().with(|s| {
+        s.option_nullable = true;
+        s.option_add_null_type = false;
+    });
     let gen = settings.into_generator();
     let schema = gen.into_root_schema_for::<MyStruct>();
     println!("{}", serde_json::to_string_pretty(&schema).unwrap());