Feature Request: remote repo selector

## Is your feature request related to a problem? Please describe.

Apparently this extension assumes the GitHub repository to use is the `git remote` named `origin`. I use a fork-based development workflow, and I haven't been using `origin` as a remote name at all, as [it's arguably unclear](https://stackoverflow.com/a/38986590). (Does it refer to my fork, or the upstream/canonical repo?) Additionally, even if the meaning of `origin` is clear, this limitation doesn't allow users to choose between multiple remotes.

## Describe the solution you'd like

Add a pulldown selector somewhere in the GITHUB ACTIONS sidepanels, to allow choosing. Save the setting for each workspace.

## Additional context

(None.)

まず、拡張機能が現在の GitHub リポジトリをどのように検出し、GITHUB ACTIONS のサイドパネルがどのように組み立てられるかを追跡します。最初に workspace 設定のフローと remote の扱いを調べます。サイドパネルのセレクターで remote の中から選択でき、その選択が workspace ごとに個別に保存されれば完了です。

索引モデルが issue の本文から書いたものです。