Refactor plugin management

Refactor plugin management:
- support multiple plugins per executable
- support restarting a plugin process in the event it terminates
- simplify plugin lifecycle management by using separate managers for
  each scope (server vs backup vs restore)

Signed-off-by: Andy Goldstein <andy.goldstein@gmail.com>

pkg/controller/download_request_controller.go

@@ -49,12 +49,14 @@ type downloadRequestController struct {
 	downloadRequestListerSynced cache.InformerSynced
 	restoreLister               listers.RestoreLister
 	restoreListerSynced         cache.InformerSynced
-	backupService               cloudprovider.BackupService
+	objectStore                 cloudprovider.ObjectStore
 	bucket                      string
 	syncHandler                 func(key string) error
 	queue                       workqueue.RateLimitingInterface
 	clock                       clock.Clock
 	logger                      logrus.FieldLogger
+
+	createSignedURL cloudprovider.CreateSignedURLFunc
 }
 
 // NewDownloadRequestController creates a new DownloadRequestController.
@@ -62,7 +64,7 @@ func NewDownloadRequestController(
 	downloadRequestClient arkv1client.DownloadRequestsGetter,
 	downloadRequestInformer informers.DownloadRequestInformer,
 	restoreInformer informers.RestoreInformer,
-	backupService cloudprovider.BackupService,
+	objectStore cloudprovider.ObjectStore,
 	bucket string,
 	logger logrus.FieldLogger,
 ) Interface {
@@ -72,11 +74,13 @@ func NewDownloadRequestController(
 		downloadRequestListerSynced: downloadRequestInformer.Informer().HasSynced,
 		restoreLister:               restoreInformer.Lister(),
 		restoreListerSynced:         restoreInformer.Informer().HasSynced,
-		backupService:               backupService,
+		objectStore:                 objectStore,
 		bucket:                      bucket,
 		queue:                       workqueue.NewNamedRateLimitingQueue(workqueue.DefaultControllerRateLimiter(), "downloadrequest"),
 		clock:                       &clock.RealClock{},
 		logger:                      logger,
+
+		createSignedURL: cloudprovider.CreateSignedURL,
 	}
 
 	c.syncHandler = c.processDownloadRequest
@@ -236,7 +240,7 @@ func (c *downloadRequestController) generatePreSignedURL(downloadRequest *v1.Dow
 		directory = downloadRequest.Spec.Target.Name
 	}
 
-	update.Status.DownloadURL, err = c.backupService.CreateSignedURL(downloadRequest.Spec.Target, c.bucket, directory, signedURLTTL)
+	update.Status.DownloadURL, err = c.createSignedURL(c.objectStore, downloadRequest.Spec.Target, c.bucket, directory, signedURLTTL)
 	if err != nil {
 		return err
 	}