Add ability to get the list of submitted jobs from gRPC JobService

As a developer building a client for monitoring running jobs via the JobService, I want the ability to get a list of jobs – particularly their job ids – so that I can use this as an entry point for getting other information about a job already offered by the JobService, such as the pipeline definition, a stream of status changes, etc. 

Currently, the JobService is only useful if you already have a valid job id.