Attempt to implement Subissues API

[DavidBoike]

Resolves #ISSUE_NUMBER

No issue yet, as I am having problems with the underlying REST API, but wanted to make my work available.

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Before the change?

• Octokit does not support subissues API

After the change?

• Octokit supports the subissues API

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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[DavidBoike]

The state of this PR is not finished, could use some xmldoc work, etc. except I'm having trouble getting it to actually work with the GitHub REST API as it's documented. I'm raising this PR to make this attempted update visible and hopefully get help on how to deal with the problems I'm having with the API.

The operations to list sub-issues and get parent issue (all the GET requests) work great, but all the mutating operations fail in various ways that don't make sense, as I can't even get them to work with CURL requests using the documented snippets in the API.

I have described what's going on in quite a bit of detail in a pre-existing discussion thread on not being able to add a sub-issue using the GitHub CLI.

Any help or guidance would be appreciated!