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.

Coder frontend plugin の API 呼び出しから始め、その isolated-dev setup と不足している backend proxy を比較します。pull request #2 で参照されている API-factory アプローチを評価し、plugin を単独でテストします。完了とは、Backstage の完全なデプロイなしでコンポーネントにデータが表示されることです。

索引モデルが issue の本文から書いたものです。