Add support for signed commits

[iliapolo]

Currently the commits made on the backported PR's are not signed, even though the original commits may have been.
In our case, this is a problem because we have the DCO check enabled that enforces signed commits on all PR's.

This is fairly common practice, and is actually a mandatory check for CNCF projects.

Would be nice if the tool could allow signing off the cherry-pick.

BTW we have just recently started using this tool (and appropriate action) in our repo - its pretty great, thanks for it! :)