CI: addlicense as linter #2558

I think it would be a good idea to use addlicense as linter in this project to ensure no copyright + license headers are missing. But I have identified the following issues to be resolved in order to use it for this:

• Use the copyright holder in check google/addlicense#164
• Ignore empty files google/addlicense#162
• Support spec files google/addlicense#163