github: Add new checklist workflow

Add a new 'checklist' workflow that checks the commit messages on pull
requests. Currently, the workflow creates a comment on the pull request
if any of these conditions are hit:

• Missing Signed-off-by
• Malformed Signed-off-by
• Bad email (i.e *noreply*)

Reviewed by: emaste, imp
Pull request: https://github.com/freebsd/freebsd-src/pull/1570

Signed-off-by: Ahmad Khalifa <ahmadkhalifa570@gmail.com>