Add standalone C++ server using cpp-httplib for OpenAPI-based APIs (#21724)

Supports:
All OpenAPI 3.x data types: primitives, arrays, enums, nullable/optional fields, nested objects
All parameter types: path, query, header, cookie, and combinations
Schema composition: allOf (inheritance), oneOf (discriminated unions), anyOf (flexible unions)
Security schemes: API key and bearer token authentication
Discriminator-based polymorphic deserialization and error handling

Provides:
Error handling for invalid JSON, type mismatches, missing/unknown discriminator, and parameter validation
Build system integration (CMake) for easy compilation and linking with required dependencies
Clear build and run instructions for local development and testing
Enables comprehensive, real-world validation of generated C++ server code against OpenAPI specifications

samples/server/petstore/cpp-httplib-server/feature-test/models/SimpleObject.cpp

@@ -0,0 +1,58 @@
+
+/**
+* This file is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+* https://openapi-generator.tech
+* Do not edit the class manually.
+*/
+#include "SimpleObject.h"
+
+namespace models {
+
+SimpleObject::SimpleObject()
+: id(0)
+, name("")
+, description("")
+{
+}
+// =========================================
+// ===== Getters/Setters =====
+// =========================================
+// ===================
+// ===== Getters =====
+// ===================
+int SimpleObject::getId() const
+{
+    return id;
+}
+std::string SimpleObject::getName() const
+{
+    return name;
+}
+std::string SimpleObject::getDescription() const
+{
+    return description;
+}
+
+// ===================
+// ===== Setters =====
+// ===================
+void SimpleObject::setId(const int& idObj)
+{
+    id = idObj;
+}
+void SimpleObject::setName(const std::string& nameObj)
+{
+    name = nameObj;
+}
+void SimpleObject::setDescription(const std::string& descriptionObj)
+{
+    description = descriptionObj;
+}
+
+
+
+
+
+
+} // namespace models
+