Expose the fields, their data types and options that exist in the project

### Please avoid duplicates

- [X] I checked [all open feature requests](https://github.com/gr2m/github-project/issues?q=is%3Aissue+is%3Aopen+label%3Afeature) and none of them matched my problem.

### What’s missing?

The ability to see the current fields configuration for a project.

### Why?

For my use case I need to list the available options for the built-in field `status` even if no items have that status set. Anyone who wants to discover how a project is configured would find the generally capability useful.

~This seems really to just be modifying `projectNodeToProperties()` to not include `fields` from `state`.~

~There could be confusion in doing this in regards to the `GitHubProject.fields` property when doing this which from my understanding is about mapping/remapping the fields given back for each item.~

_I ignorantly thought this was all fields in the project, not those just mapped through with options._

### Alternatives you tried

Could have written my own query but since everything was already loaded I just patched `projectNodeToProperties()`.

### Would you be interested in contributing the feature?

- [X] yes

projectNodeToProperties() と GitHubProject.fields プロパティから始め、既存のクエリがプロジェクトデータをどのように読み込んでいるかを確認します。完成した機能では、現在のプロジェクトフィールド、そのデータ型、利用可能なオプションを公開する必要があります。現在どのアイテムも使用していない場合は、status などの組み込みフィールドのオプションも含めます。

索引モデルが issue の本文から書いたものです。