service: Update DlcArtifactsMetadata to include ID

RBS + GE to ingest this value, rather than parsing IDs out from
arbitrary URIs.

BUG=b:286327155
TEST=./run_tests

Change-Id: I75aa813c75cba4efc2afc05d293fc8a3bb51b40a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4811927
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Benjamin Shai <bshai@google.com>
Auto-Submit: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/api/controller/dlc_unittest.py b/api/controller/dlc_unittest.py
index ac5f969..f53b048 100644
--- a/api/controller/dlc_unittest.py
+++ b/api/controller/dlc_unittest.py
@@ -50,6 +50,7 @@
                     image_hash="deadbeef",
                     image_name="dlc.img",
                     uri_path="gs://some/uri/prefix/for/dlc-1",
+                    identifier="dlc-1",
                 )
             ],
         )