1. Proposed title of this feature request
Allow setting a proxy for connecting to the cluster at login.

2. What is the nature and description of the request?
With this change, the given proxy will be used set and used during the cluster config creation (ie first login):

$ ./oc login -x https://proxy.example.com https://cluster.example.com:6443 --web
Opening login URL in the default browser: ...
Login successful.

You have access to 298 projects, the list has been suppressed. You can list all projects with 'oc projects'

Using project "default".

$ head ~/.kube/config
apiVersion: v1
clusters:
- cluster:
    proxy-url: https://proxy.example.com     <---------
    server: https://cluster.example.com:6443
  name: cluster-example-com:6443
...

An upstream PR exists already add proxy-url flag to login cmd #1852

3. Why does the customer need this? (List the business requirements here)
Setting a proxy globally works but we need to use a specific proxy only for connection to our clusters.

4. List any affected packages or components.
oc client