Feature: use proxy-url from cluster in kubeconfig if set

[veith4f]

Description of your changes

Clusters in kubeconfig may have a field proxy-url that needs to be respected to reach the specified endpoints.
Currently, client.go in the provider does not respect this.

struct rest.Config created in client.go offers a field proxy:func(*http.Request) (*url.URL, error) that may be used to this end.

How has this code been tested

Just compile/deploy-local as of now. Want to see if there is interest.

[veith4f]

Was now successfully deployed, packaged, installed and tested in our environment.

Kubernetes Object was correctly provisioned.

[mcfarlanem]

Compiled, packaged, deployed, installed, and tested provider-kubernetes and provider-helm based on this branch. Everything works as expected with proxy-url in private EKS cluster and without proxy-url in public EKS cluster.

Thanks @veith4f for this work. Can we get this contribution merged?

See #347.

[mcfarlanem]

@veith4f @turkenf it's disappointing we haven't got this contribution merged yet. Since my last post I have literally deployed hundreds of clusters using this. IMHO this is a better solution than using env vars at the pod level, not to mention it taking advantage of core Kubernetes capability.

[veith4f]

How bout it guys. Can we get this merged?