Follow-up from "Add pipelineSchedulePlay mutation"

The following discussions from !97857 (closed) should be addressed:

Completed in: !177181 (merged) & !177203 (merged)

• @allison.browne started a discussion:

  Suggestion(non-blocking): This condition will happen when the PipelineScheduleService returns null. We could have that service return a service response error instead which is what we prefer to return from a service in a condition that would return an error to the user.

• @allison.browne started a discussion:

  Follow-ups(no change needed): This is mostly a note so that I remember to make follow up issues prior to merge. No changes are needed in this MR.

  1. Since we now practice defense in depth we should check that the user has permission to play the pipeline schedule here. https://docs.gitlab.com/ee/development/permissions.html#where-should-permissions-be-checked
  2. Based on the in progress software design guides, we should re-name this to Ci::PipelineSchedule::PlayService
  3. To keep the implementation the same across the REST and graphql API we should use this service in both endpoints. Currently, we call the RunPipelineScheduleWorker directly.