Add proxy-url command line option

[kriswuollett]

What would you like to be added:

Add proxy-url command line option to override cluster setting in selected context config.

Why is this needed:

Golang doesn't support HTTPS_PROXY for localhost addresses as noted in #1653. Avoids the need to statically set proxy urls for each of the clusters defined in a config file.

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[xyz-li]

/assign

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[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
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• Mark this issue as fresh with /remove-lifecycle rotten
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/lifecycle rotten