Add infrastructure for argocd addon

[nirs]

Add additional drenv infrastructure needed for the argocd addon, but can be useful for other addons or future code.

• drenv.temporary_kubeconfig() - creating a temporary kubeconfig from the default kubeconfig, and returning an environment dict pointing to the kubeconfig.
• support env= argument in commands.run() and commands.watch() - useful for running commands with modified environment variables.

[par97]

I have used with this PR code and it works well with argocd operations.