Implement GetActualInstanceValue method (#20663)

Author: Sander0542

modules/openapi-generator/src/main/resources/go/client.mustache

@@ -137,6 +137,10 @@ func typeCheckParameter(obj interface{}, expected string, name string) error {
 
 func parameterValueToString( obj interface{}, key string ) string {
 	if reflect.TypeOf(obj).Kind() != reflect.Ptr {
+		if actualObj, ok := obj.(interface{ GetActualInstanceValue() interface{} }); ok {
+			return fmt.Sprintf("%v", actualObj.GetActualInstanceValue())
+		}
+
 		return fmt.Sprintf("%v", obj)
 	}
 	var param,ok = obj.(MappedNullable)

modules/openapi-generator/src/main/resources/go/model_oneof.mustache

@@ -145,4 +145,16 @@ func (obj *{{classname}}) GetActualInstance() (interface{}) {
 	return nil
 }
 
+// Get the actual instance value
+func (obj {{classname}}) GetActualInstanceValue() (interface{}) {
+{{#oneOf}}
+	if obj.{{#lambda.type-to-name}}{{{.}}}{{/lambda.type-to-name}} != nil {
+		return *obj.{{#lambda.type-to-name}}{{{.}}}{{/lambda.type-to-name}}
+	}
+
+{{/oneOf}}
+	// all schemas are nil
+	return nil
+}
+
 {{>nullable_model}}