Credentials to user to scan the selected project and its git repositories as well as checkout sources.
At the moment Only "username + password" credentials are supported. Other types won't be proposed (esp. API key cannot be used) because Jenkins should be able to do both REST API calls and git http operations and only username/password with Basic Authentication can do that for now.
If the project contains private repositories or if the project itself is private, given credentials should have access to this project.