oc login should support proxy-url option

The oc login command does not currently support the proxy-url option supported in the kubeconfig:

https://kubernetes.io/docs/reference/config-api/kubeconfig.v1/

Ideally it would be possible to pass something like --proxy-url=http://squid.redhat.com:3128 and oc login would use the proxy setting the same as if HTTPS_PROXY were set and configure the proxy-url setting for the cluster in the kubeconfig.

Current behavior is unfortunate because if proxy-url is manually set in the kubeconfig it works for normal commands, but when oc login is run again it not only doesn't work, but when HTTPS_PROXY environment variable is set it removes the proxy-url setting, breaking functionality for other oc commands.

This functionality is particularly important for cases where multiple clusters are managed through oc and some should use a proxy, some not, or different proxy servers should be used for different clusters.

Start at the oc login command and its kubeconfig handling, using the linked Kubernetes kubeconfig reference to confirm the proxy-url behavior. Support a --proxy-url value that configures the cluster without losing an existing setting, and verify behavior for proxy and non-proxy clusters, including interaction with HTTPS_PROXY.