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

Start with projectNodeToProperties() and the GitHubProject.fields property, then inspect how the existing query loads project data. The completed feature should expose the current project fields, their data types, and available options, including options for built-in fields such as status when no item currently uses them.

Scritto dal modello di indicizzazione a partire dal testo della issue.