Support for checking a PR’s auto-merge status; removing a label from an issue; `perPage` parameter when listing PRs.

[jasdev]

Some changes we needed at BCNY that I figured we can upstream (cc @pietbrauer). :D

[pietbrauer]

Hey, thanks for the PR. To merge this it would be nice to:

• have the interfaces with completion blocks added
• remove the changes to the package.swift and xcodeproj
• add some tests

[jasdev]

Will re-open when I get time to address https://github.com/nerdishbynature/octokit.swift/pull/168#issuecomment-1546676779 🙏🏽