feat: add support for subissues

[paulpach]

Adds support for subissues to the API.

Resolves #327

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Before the change?

No way to get parent, subissues or subissues summary

After the change?

The issues have a Parent, Subissues and subissues summary fields

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[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! 🚀

[Lamparter]

Thank you for this change @paulpach!

[Lamparter]

Sorry, I don't have write access to this specific repo.

[DavidBoike]

So…who does have write access to this repo? Are the Monday/Friday PR reviews happening as advertised?

[Lamparter]

Unfortunately not. I can ping the SDK team but I'm afraid that won't warrant a response due to the inactivity of the project.