feat(scaffolder): experimental flag to wait for task completions

Enabling the `EXPERIMENTAL_gracefulShutdown` flag in the scaffolder config will make the
scaffolder block the shutdown process until all running tasks have completed. This is useful
when there is a need to ensure that all tasks have completed before the scaffolder is shut down.

Signed-off-by: Hellgren Heikki <heikki.hellgren@op.fi>

plugins/scaffolder-backend/config.d.ts

@@ -41,6 +41,11 @@ export interface Config {
      */
     concurrentTasksLimit?: number;
 
+    /**
+     * Tries to wait for tasks to finish during SIGTERM before shutting down the TaskWorker.
+     */
+    EXPERIMENTAL_gracefulShutdown?: boolean;
+
     /**
      * Sets the tasks recoverability on system start up.
      *