Update Coder plugin to work in isolated dev mode

[Parkreiner]

One of the big points that Backstages stresses in their docs is that plugins should be able to be spun up in isolation. You shouldn't need to bring in a whole Backstage deployment to test component changes.

The Coder plugin is a frontend plugin that relies on a proxied endpoint to the main Coder API. It can be spun up in isolation, but because it's missing a backend proxy, the component will never populate with data.

As mentioned here, API factories might do the trick, but they're not as well-documented, and they still require testing. Depending on how much needs to change, we might need to revamp our plugin's API calls, too.