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

Beginne mit projectNodeToProperties() und der Eigenschaft GitHubProject.fields und untersuche anschließend, wie die bestehende Abfrage Projektdaten lädt. Das fertige Feature sollte die aktuellen Projektfelder, ihre Datentypen und verfügbaren Optionen bereitstellen, einschließlich Optionen für integrierte Felder wie status, wenn derzeit kein Element sie verwendet.

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