add proxy-url flag to login cmd

[Alex2242]

Allow setting a proxy for connecting to the cluster at login.

Setting a proxy globally works but we need to use a specific proxy only for connection to our clusters.

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
...

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Alex2242
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[openshift-ci]

Hi @Alex2242. Thanks for your PR.

I'm waiting for a openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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