Add option for --signoff

[michaelmior]

This should fix #2408

[mzuehlke]

Looks good, thanks

[michaelmior]

@mzuehlke Thanks! Would there be any chance of having this enabled for the public instance? If preferred, I can add a repo config option as an alternative.

[mzuehlke]

Enabling this by default for all repos is something I wouldn't prefer.
But having this as an option in the per repo config file (https://github.com/scala-steward-org/scala-steward/blob/main/docs/repo-specific-configuration.md), to turn this on, sounds like a good idea to me 👍

[michaelmior]

@mzuehlke That makes sense. See #3432