Allow brew bump-formula-pr to create draft PRs

[rgoldberg]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

Allow brew bump-formula-pr to create draft PRs by supporting a --draft flag.

What is the motivation for the feature?

This would allow automated release workflows to submit a formula bump to homebrew-core, but ensure that maintainers do not approve or merge it before the releaser is ready.

This allows a releaser / automated release process to detect formula bump problems while, e.g., a GitHub Release is still a draft. The releaser / automated release process can inspect, then potentially fix, then publish a release for a validated draft, then change the homebrew-core PR from a draft to an actual submission.

If a release suffers from unfixable problems, then the formula bump draft PR can be closed without fear of it having been approved, while, e.g., the GitHub Release draft can be deleted.

How will the feature be relevant to at least 90% of Homebrew users?

It won't be directly used by users, but rather mainly by developers who bump the formula for new releases of their own software.

It seems useful & relatively easy-to-implement.

What alternatives to the feature have been considered?

• Creating a non-draft bump PR while a new release is still only a draft, then using gh pr ready --undo or equivalent to convert the PR to a draft PR.

• Creating a non-draft bump PR only after publishing a new release.