override postProcessModelProperty to set vendor extension

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractGoCodegen.java

@@ -584,6 +584,17 @@ private void setExportParameterName(List<CodegenParameter> codegenParameters) {
         }
     }
 
+    @Override
+    public void postProcessModelProperty(CodegenModel model, CodegenProperty property) {
+        // The 'go-experimental/model.mustache' template conditionally generates accessor methods.
+        // For primitive types and custom types (e.g. interface{}, map[string]interface{}...),
+        // the generated code has a wrapper type and a Get() function to access the underlying type.
+        // For containers (e.g. Array, Map), the generated code returns the type directly. 
+        if (property.isContainer || property.isFreeFormObject || property.isAnyType) {
+            property.vendorExtensions.put("x-golang-is-container", true);
+        }
+    }
+
     @Override
     public Map<String, Object> postProcessModels(Map<String, Object> objs) {
         // remove model imports to avoid error