[FEAT]: access to the subissues api

[paulpach]

Describe the need

Our org uses the subissues preview feature

I am not able to access the extra fields with octokit.graphql.net. There does not appear to be a way to pass the preview feature header or access the "parent" field.

related to #130

SDK Version

No response

API Version

No response

Relevant log output

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labeled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[paulpach]

This went GA not too long ago. There is no need to add a header anymore.

However, I still need the Issue.Parent and Issue.SubIssues field.