I code a small replacement for the postman mock servers and I see that only the pro version of http toolkit come with this feature. If you are interested in a simple and small replacement:
https://github.com/sgrodriguez/yams
Last year I built a go lib call DDT (Dynamic decision tree)
DDT allows building custom decision trees based in a set of defined rules, programmatically or from json.
When making a decision, it allows adding a pre-processing stage to the input before comparing it with the following possible branches of the tree.