Update Coder plugin to work in isolated dev mode

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](https://github.com/coder/backstage-plugins/pull/2#pullrequestreview-1883966805), 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.