Add decommissioned event type reason to agency vehicle_event schema

Author: thekidder

agency/post_vehicle_event.json

@@ -142,7 +142,8 @@
             },
             "event_type_reason": {
               "enum": [
-                "missing"
+                "missing",
+                "decommissioned"
               ]
             }
           }
@@ -247,4 +248,4 @@
     }
   },
   "additionalProperties": false
-}
+}