feat: add default_query_job_config property and property setter to BQ client (#1511)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [x] Make sure to open an issue as a [feature request](https://togithub.com/googleapis/python-bigquery/issues/1512) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

Fixes 
- [feature request](https://togithub.com/googleapis/python-bigquery/issues/1512)🦕
- [internal bug](https://b.corp.google.com/issues/271044948)

Author: chelsea-lin

google/cloud/bigquery/client.py

@@ -266,6 +266,17 @@ def location(self):
         """Default location for jobs / datasets / tables."""
         return self._location
 
+    @property
+    def default_query_job_config(self):
+        """Default ``QueryJobConfig``.
+        Will be merged into job configs passed into the ``query`` method.
+        """
+        return self._default_query_job_config
+
+    @default_query_job_config.setter
+    def default_query_job_config(self, value: QueryJobConfig):
+        self._default_query_job_config = copy.deepcopy(value)
+
     def close(self):
         """Close the underlying transport objects, releasing system resources.
 

tests/unit/test_client.py

@@ -413,6 +413,19 @@ def test__get_query_results_hit(self):
         self.assertEqual(query_results.total_rows, 10)
         self.assertTrue(query_results.complete)
 
+    def test_default_query_job_config(self):
+        from google.cloud.bigquery import QueryJobConfig
+
+        creds = _make_credentials()
+        http = object()
+        client = self._make_one(project=self.PROJECT, credentials=creds, _http=http)
+        self.assertIsNone(client.default_query_job_config)
+
+        job_config = QueryJobConfig()
+        job_config.dry_run = True
+        client.default_query_job_config = job_config
+        self.assertIsInstance(client.default_query_job_config, QueryJobConfig)
+
     def test_get_service_account_email(self):
         path = "/projects/%s/serviceAccount" % (self.PROJECT,)
         creds = _make_credentials()