Support updating pull request draft status (#3104)

Adds:
- `PullRequest.convert_to_draft` to mark a pull request as draft
- `PullRequest.mark_ready_for_review` to mark a pull request as ready
for review

Fixes #1461
Fixes #1861
Fixes #2989

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>