jobqueue: start moving API types to json.go

Similar to `weldr/json.go`, this file will contain structs that are
shared between client and server and only meant to be used to
serialize from and to JSON.

This immediate change allows us to make fields in `Job` private in a
future commit.

internal/jobqueue/api.go

@@ -102,7 +102,7 @@ func (api *API) addJobHandler(writer http.ResponseWriter, request *http.Request,
 
 	writer.WriteHeader(http.StatusCreated)
 	// FIXME: handle or comment this possible error
-	_ = json.NewEncoder(writer).Encode(Job{
+	_ = json.NewEncoder(writer).Encode(addJobResponse{
 		ID:           nextJob.ComposeID,
 		ImageBuildID: nextJob.ImageBuildID,
 		Manifest:     nextJob.Manifest,
@@ -130,7 +130,7 @@ func (api *API) updateJobHandler(writer http.ResponseWriter, request *http.Reque
 		return
 	}
 
-	var body JobStatus
+	var body updateJobRequest
 	err = json.NewDecoder(request.Body).Decode(&body)
 	if err != nil {
 		statusResponseError(writer, http.StatusBadRequest, "invalid status: "+err.Error())