[REQ][Rust] Add additionalProperties to the generated code

Is your feature request related to a problem? Please describe.

Some openAPI specs do not contain every property they actually return. Right now it is not possible to access those properties

Describe the solution you'd like

The solution would be to include all undefined properties in the generated code, by modifying the model.mustache file like this:

--- /proc/self/fd/14	2025-03-25 10:57:36.081920827 +0100
+++ templates/model.mustache	2025-03-25 09:53:47.536340813 +0100
@@ -148,6 +148,8 @@
     ### Option End (and trailing comma)
     }}{{#isNullable}}>{{/isNullable}}{{^required}}>{{/required}},
 {{/vars}}
+    #[serde(flatten)]
+    pub additional_properties: std::collections::HashMap<String, serde_json::Value>,
 }
 
 impl {{{classname}}} {
@@ -169,6 +171,7 @@
             {{#vars}}
             {{{name}}}{{^required}}: None{{/required}}{{#required}}{{#isModel}}{{^avoidBoxedModels}}: {{^isNullable}}Box::new({{{name}}}){{/isNullable}}{{#isNullable}}if let Some(x) = {{{name}}} {Some(Box::new(x))} else {None}{{/isNullable}}{{/avoidBoxedModels}}{{/isModel}}{{/required}},
             {{/vars}}
+            additional_properties: std::collections::HashMap::new(),
         }
     }
 }

Start with templates/model.mustache and compare the Rust model fields and constructor shown in the issue. Generate a Rust model from a spec containing undeclared response properties, then verify the generated code exposes those properties and initializes the added field correctly.