Support nullable fields in the Rust generator (#2353)

* Use Option for nullable fields in Rust (closes #2337)

* Update Rust's petstore

samples/client/petstore/rust/src/models/inline_object.rs

@@ -0,0 +1,32 @@
+/*
+ * OpenAPI Petstore
+ *
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * OpenAPI spec version: 1.0.0
+ * 
+ * Generated by: https://openapi-generator.tech
+ */
+
+
+#[allow(unused_imports)]
+use serde_json::Value;
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct InlineObject {
+    /// Updated name of the pet
+    #[serde(rename = "name")]
+    pub name: Option<String>,
+    /// Updated status of the pet
+    #[serde(rename = "status")]
+    pub status: Option<String>,
+}
+
+impl InlineObject {
+    pub fn new() -> InlineObject {
+        InlineObject {
+            name: None,
+            status: None,
+        }
+    }
+}

samples/client/petstore/rust/src/models/inline_object_1.rs

@@ -0,0 +1,32 @@
+/*
+ * OpenAPI Petstore
+ *
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * OpenAPI spec version: 1.0.0
+ * 
+ * Generated by: https://openapi-generator.tech
+ */
+
+
+#[allow(unused_imports)]
+use serde_json::Value;
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct InlineObject1 {
+    /// Additional data to pass to server
+    #[serde(rename = "additionalMetadata")]
+    pub additional_metadata: Option<String>,
+    /// file to upload
+    #[serde(rename = "file")]
+    pub file: Option<&std::path::Path>,
+}
+
+impl InlineObject1 {
+    pub fn new() -> InlineObject1 {
+        InlineObject1 {
+            additional_metadata: None,
+            file: None,
+        }
+    }
+}