Add scaffolding for cron trigger support

database/db.go

@@ -189,6 +189,16 @@ CREATE TABLE pipeline_refs (
         FOREIGN KEY(pipeline_id)
             REFERENCES pipelines(id)
 );
+
+CREATE TABLE crons (
+	id              UUID PRIMARY KEY,
+	pipeline_id     UUID NOT NULL,
+	cron            TEXT NOT NULL,
+
+	CONSTRAINT fk_pipeline_id
+        FOREIGN KEY(pipeline_id)
+            REFERENCES pipelines(id)
+);
 `
 
 	_, err := conn.Exec(context.Background(), createTablesQuery)