Proxy support is broken

Describe the bug

Proxy support was introduced in #1215, but it is currently broken.

To Reproduce

Steps to reproduce the behavior:

1. Somehow make api.netlify.com unavailable, e.g. by being on a closed network where a proxy is required. For reproduction purposes, one can force this by modifying /etc/hosts and pointing api.netlify.com to a local container / VM where we spin up a trivial HTTP server that simply stalls on any incoming request.
2. Have a proxy server set up, e.g. mitmproxy or Charles.
3. Run any command through a proxy, e.g. HTTP_PROXY=http://localhost:8888 netlify login.

At this point netlify-cli would simply stall, and if you monitor the proxy server you'll notice that no request is going through it.

Configuration

N/A.

Expected behavior

Any network request originating from netlify-cli should be proxied.

CLI Output

N/A.

Additional context

This happens because getConfig happens before the proxy is even configured (with getAgent)

https://github.com/netlify/cli/blob/7a55e9871059ac656096735d1c0369b4d473e07e/src/utils/command.js#L46-L57

getConfig calls resolveConfig which is @netlify/config.

https://github.com/netlify/cli/blob/7a55e9871059ac656096735d1c0369b4d473e07e/src/utils/command.js#L4

Looking at @netlify/config's resolveConfig: https://github.com/netlify/build/blob/6813d73a21a6c4a6b99576c5b7bc60fae5d8bac4/packages/config/src/main.js#L32-L36 we see that there are API requests made at this stage unless offline is true. These requests obviously aren't going through the proxy.

I guess to actually fully support proxies you need to add proxy support to @netlify/config as well.

Knowing the cause of the issue, here's a risky workaround btw: just set offline to true in src/utils/command.js:

-    const cachedConfig = await this.getConfig({ cwd, state, token, ...apiUrlOpts })
+    const cachedConfig = await this.getConfig({ cwd, state, token, offline: true, ...apiUrlOpts })

One can hot edit this directly into node_modules/netlify-cli/src/utils/command.js. (Don't complain to me if anything breaks, obviously.)

Related: I believe #2016 is exactly this problem.

Start in src/utils/command.js, where getConfig runs before getAgent configures the proxy, then trace resolveConfig in @netlify/config at packages/config/src/main.js. Reproduce with HTTP_PROXY=http://localhost:8888 netlify login and a stalled api.netlify.com endpoint. Done means the initial configuration requests use the proxy without breaking normal or offline behavior.