Expose WorkerRun summaries in all WorkerRun endpoints

WorkerRun.summary is only available on ML results that use a WorkerRun, for example in RetrieveElement, but can be useful when accessing the WorkerRun itself. Some of the modals used for frontend#1267 (closed) have to refer to specific worker runs in the UI and end up reimplementing worker run summaries. We could add the summary in WorkerRunSerializer to make it available everywhere, and it won't change anything in terms of performance since the summary is already implicitly fetched everywhere by Django.