Auto-style and auto-roxygenize for foreign PRs

[krlmlr]

This can be built, I think, but needs a good test setup for fast iteration.

Idea:

• In the commit action, upload a patch and the PR information to an artifact
• Pick up this artifact in the rcc-status action
• Create PR in the rcc-status action
• Can it be merged into the foreign PR? If not, post a comment.

[krlmlr]

To be solved with reviewdog.