Added proxy auth support to model.Endpoint

[hughneale]

Adding support for https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Proxy-Authorization. Is this ok? Do you want me to update the DSL spec first?

Just lets HTTP callers auth against resources that support the proxy auth header like Google's IAP

[ricardozanini]

@hughneale some languages/frameworks deal with this internally already. I'm not familiar with the details from Go HTTP client. I recognize the need of it, but maybe we could delegate to the internal HTTP client?

[ricardozanini]

I mean, is proxy authentication on the same level and the application authz? That I have questions about.