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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