Add test case for update volume backup

Update volume backup is a new api implemented in volume v3 (since 3.9).
This patch is to add test case for this new feature.

Including:

[1] Add v3 backups_client as a library
[2] Add update backup api to v3 backups_client
[3] Add unit tests for update backup api
[4] Add test case: test_update_backup
[5] Add release note

Change-Id: Id50fcdbc41e9e170f3d467789f80c28bac01d434
diff --git a/releasenotes/notes/add-update-backup-api-to-v3-backups-client-e8465b2b66617dc0.yaml b/releasenotes/notes/add-update-backup-api-to-v3-backups-client-e8465b2b66617dc0.yaml
new file mode 100644
index 0000000..7cd6887
--- /dev/null
+++ b/releasenotes/notes/add-update-backup-api-to-v3-backups-client-e8465b2b66617dc0.yaml
@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    Define v3 backups_client for the volume service as a library interface,
+    allowing other projects to use this module as a stable library without
+    maintenance changes.
+    Add update backup API to v3 backups_client library, min_microversion
+    of this API is 3.9.
+
+    * backups_client(v3)