Enable support for creating draft PRs

[luislew]

draft is a boolean parameter on the create PRs API: https://docs.github.com/en/rest/reference/pulls#create-a-pull-request--parameters

Maybe we could add support for shepherd pr --draft? This is useful in cases where you want to open a lot of draft PRs to exercise CI builds without any intention of merging the PRs.

[aorinevo]

That's an interesting use case.

While the use case assumes PRs will never be merged and exclusively used to exercise CI, it's likely some users will want to transition the PRs to ready-for-review state. To that end, how would a user transition PRs opened in draft mode via shepherd pr --draft out of draft state?

If you're interested in contributing this change, I'm happy to review.