Add `rebase-strategy`-like option for the `/pr` action variant

[ericcornelissen]

Relates to #180

Summary

Add support for an option like the Dependabot rebase-strategy option to allow users to control how Pull Request get updated.

The current behavior is that Pull Requests are automatically rebased unless it is detected to be changed by the user (i.e. there's additional commits on the branch).

I can see several behaviors that might be desirable:

• Never rebase (this would also improve the usability of the "One at the Time" recipe use case).
• Rebase if there are no changes (current)
• Always rebase (peter-evans/create-pull-request's default)