Third party provider support for out of cluster config

[clux]

Certainly nice to have so we don't have to impersonate service accounts with their tokens directly outside the cluster.

I can see there's some partial support upstream via:

• Add auth_provider for cloud ynqa/kubernetes-rust#16
• Add credential-plugin authentication support ynqa/kubernetes-rust#15
• Allow context, cluster, and user names to be specified ynqa/kubernetes-rust#17

and will try to port it. That said:

It would be really awesome to have a dedicated crate for the kubeconfig. Preferably one that doesn't export a http client to use with api calls. That way this library can be purely an API abstraction and a completely disjoint approach to ynqa's version.

[clux]

Port branch available, and it doesn't break service account auth at least, but I also have no way of testing it here at the moment. A dedicated crate if someone is willing to help out with a better testing strategy would be amazing.

[clux]

#20 merged, the support is probably not great still, but at least the logic is there and people report EKS working.

[clux]

Forgot to close this :/