Allow fetch schema from Kubernetes API rather than source code

[busser]

Let me start by saying this plugin is a great idea!

The plugin currently fetches the Kubernetes OpenAPI spec from the Kubernetes source code on GitHub. This means the spec only contains native types.

Fetching the OpenAPI spec from a Kubernetes cluster's API would add any custom resource definitions. I would love for this plugin to allow ChatGPT to generate valid Istio VirtualService resources, for example.