Allow creating the PR in draft mode

[tom-sherman]

In our team we assign all pull requests that are opened round-robin style to one of the team members, so that every PR has a defined "owner" and point of contact.

The PRs that changesets open can complicate this, because there's nothing to actually own or action until we decide to do a release. It leads to pointless github notifications and the person being assigned needing to unsubscribe and un-assign themselves.

We also have a way of working of PRs that are open, if they pass CI, can be merged. The changeset PR is an exception to this though.

Both of these problems can be solved by adding an option for the action to create the PR in draft mode. Then when it's time to release, someone (or an API call) can mark it as not draft - assigning a team member who can review and release.

[Andarist]

I'd accept a PR implementing this as an option

[tom-sherman]

Happy to do that!