feat: grant users access to query audit logs (#148)

This PR updates the Datum Cloud Viewer role to allow querying audit log
data from the new activity system. See
datum-cloud/activity#12 for details.

---

Relates to https://github.com/datum-cloud/engineering/issues/90

Author: scotwells

config/assignable-organization-roles/roles/datum-cloud-viewer.yaml

@@ -24,3 +24,5 @@ spec:
       namespace: milo-system
     - name: iam-user-invitations-reader
       namespace: milo-system
+    - name: activity.miloapis.com-audit-log-querier
+      namespace: milo-system