Proxy Support for Plugins

*Bug*
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This is a bug request 👍I have updated to the latest version of the heroku CLI 👍 we use homebrew to install the plugin.

What is the current behaviour?
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Right now, whenever we use the Heroku CLI it works perfectly. 👍 However, when we try and use plugins, for example, the [heroku slugs](https://github.com/heroku/heroku-slugs) plugin, it doesn't seem to be honouring the proxy, as the standard CLI does?

Now, I thought this was because when looking in the source code for the [heroku slugs](https://github.com/heroku/heroku-slugs/blob/master/lib/download.js) there is no proxy support. Now, I don't know how plugins are meant to work. But when using this plugin, it doesn't seem to be honouring the proxy. When we use any normal CLI commands, we can see some logs in our proxy, but when we use the plugin commands, it doesn't seem to be hitting the proxy, no logs.

What is the expected behaviour?
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I would expect plugins by default to use the in-built network settings in the CLI. Or, for plugins to have to reference an exported proxy function which can be called within the plugin? I am curious to see if this is expected to work like this, or not. Just thought I would log this here and get thoughts 👍

We are indeed behind a corporate firewall, the way we get out of our network is via the proxy. 👍

I did log an issue in the [heroku slugs](https://github.com/heroku/heroku-slugs/issues/16) repository but was told to log one in this repo 👍