feat: make proxy settings configurable

[jawnsy]

• Add coderd proxy settings for outbound connections
• Move utility functions into separate file
• Extract LoadChart out of TestDeployment tests to avoid repeatedly
  loading the chart for parallel tests
• Refactor Render function to accept a function closure, as with
  MustRender, to make the function signatures more consistent
• Add utility function to convert hard-coded environment variables
  into a map

[jawnsy]

I tested this using an air-gapped installation. The cluster does not have external network access, and the only path to the Internet is a dual-homed host with a proxy installed.

Using the following values:

coderd:
  proxy:
    http: http://10.182.0.2:8888
    exempt: ghcr.io,cluster.local

I verified that:

• In-cluster traffic works correctly (coderd can communicate with kubernetes API, which does not use the proxy)
• Importing images from ghcr.io times out, since we try to connect directly and the system is air-gapped
• Importing images from GCR works correctly (since images use the internal Private Google Access network)
• Importing images from Docker Hub works correctly (they can be tracked by Coder, but not actually pulled, since the worker node is not configured to pull through the proxy)

[sc-19357]

[shortcut-integration]

This pull request has been linked to Shortcut Story #19357: Allow coderd to use a corporate proxy for outbound (license and container) traffix.

[jawnsy]

For posterity, this article surveys the different environment variable options and how they work: https://about.gitlab.com/blog/2021/01/27/we-need-to-talk-no-proxy/