[CppRest] Add Object and fix modelbase includes (#6905)

* Objects are represented by `ModelBase.h`, not `Object.h` which does not exist.

* Use correct relative imports for files in the `model` and `api` folders.

* [CppRest] Update cpprest petstore client sample.

* Implement `Object` class to support arbitrary types.

* [CppRest] Update cpprest petstore client sample.

* [CppRest] Add newly generated files to petstore client sample.

* [CppRest] Add `Object` to CMakeLists.

samples/client/petstore/cpprest/ModelBase.cpp

@@ -367,6 +367,12 @@ double ModelBase::doubleFromHttpContent(std::shared_ptr<HttpContent> val)
     return result;
 }
 
+web::json::value ModelBase::valueFromHttpContent(std::shared_ptr<HttpContent> val)
+{
+    utility::string_t str = ModelBase::stringFromHttpContent(val);
+    return web::json::value::parse(str);
+}
+
 }
 }
 }