Feature Request: Initialization Script

Currently, if we want to test a markdown block that depends on certain dependencies e.g. [installing `krew`](https://github.com/Azure/kubectl-aks/blob/main/.github/workflows/kubectl-aks.yml#L284), we have to perform these steps either via Makefile or as [additional step](https://github.com/Azure/kubectl-aks/blob/main/.github/workflows/kubectl-aks.yml#L284) in CI.

It would be great if we have a way to specify an initialization script for a document (similar to how we can specify custom vars) to be able to handle dependencies in consistent way.