Add scheduling_type to CiJob, Pipeline in GraphQL

For adding the Needs view to the pipeline graph (see &4509 (closed), #298973 (closed)), we want to only show the dropdown when needs keywords are used in a pipeline.

In the REST API, we have scheduling_type on the job to distinguish between jobs where needs is empty because it uses stages and where it is empty because it doesn't have any dependencies. I would like to add this to the CiJob type.

If it is also possible to calculate on the backend whether all jobs are stage-based (and therefore the whole pipeline is) and report that on the Pipeline type, that would be amazing, but we can calculate on the frontend if necessary.