chore: Update discovery artifacts (#1389)

## Discovery Artifact Change Summary:
feat(bigquery): update the api https://github.com/googleapis/google-api-python-client/commit/086d714317a73331fcfdf4027496c3b36354955f
diff --git a/googleapiclient/discovery_cache/documents/bigquery.v2.json b/googleapiclient/discovery_cache/documents/bigquery.v2.json
index e026908..a4d1c49 100644
--- a/googleapiclient/discovery_cache/documents/bigquery.v2.json
+++ b/googleapiclient/discovery_cache/documents/bigquery.v2.json
@@ -1683,7 +1683,7 @@
       }
     }
   },
-  "revision": "20210515",
+  "revision": "20210518",
   "rootUrl": "https://bigquery.googleapis.com/",
   "schemas": {
     "AggregateClassificationMetrics": {
@@ -6525,6 +6525,10 @@
           "description": "[Required] A query that BigQuery executes when the view is referenced.",
           "type": "string"
         },
+        "useExplicitColumnNames": {
+          "description": "True if the column names are explicitly specified. For example by using the 'CREATE VIEW v(c1, c2) AS ...' syntax. Can only be set using BigQuery's standard SQL: https://cloud.google.com/bigquery/sql-reference/",
+          "type": "boolean"
+        },
         "useLegacySql": {
           "description": "Specifies whether to use BigQuery's legacy SQL for this view. The default value is true. If set to false, the view will use BigQuery's standard SQL: https://cloud.google.com/bigquery/sql-reference/ Queries and views that reference this view must use the same flag value.",
           "type": "boolean"