I’m not familiar at all with Tyk, but it seems like you are maybe trying to create a middleware plugin. In addition to those links Jeff posted did you check this out?
https://tyk.io/docs/plugins/supported-languages/golang/
It also looks like you were asking for a go module to help you work with JWTs. This is well maintained and I’ve used it on a few projects:
So, follow the Tyk docs to create a middleware, then parse the JWT using that library and set a header or do something with the response based on the expiration date.