Refactor QueryJob.query to read from resource dictionary (#7763)

This explicitly documents the QueryJob.query property and also aligns it
with the other resource properties in the BigQuery client.

bigquery/google/cloud/bigquery/job.py

@@ -2357,7 +2357,10 @@ def __init__(self, job_id, query, client, job_config=None):
         if job_config.use_legacy_sql is None:
             job_config.use_legacy_sql = False
 
-        self.query = query
+        _helpers._set_sub_prop(
+            self._properties, ["configuration", "query", "query"], query
+        )
+
         self._configuration = job_config
         self._query_results = None
         self._done_timeout = None
@@ -2424,6 +2427,17 @@ def priority(self):
         """
         return self._configuration.priority
 
+    @property
+    def query(self):
+        """str: The query text used in this query job.
+
+        See:
+        https://cloud.google.com/bigquery/docs/reference/rest/v2/jobs#configuration.query.query
+        """
+        return _helpers._get_sub_prop(
+            self._properties, ["configuration", "query", "query"]
+        )
+
     @property
     def query_parameters(self):
         """See
@@ -2516,7 +2530,6 @@ def to_api_repr(self):
     def _copy_configuration_properties(self, configuration):
         """Helper:  assign subclass configuration properties in cleaned."""
         self._configuration._properties = copy.deepcopy(configuration)
-        self.query = _helpers._get_sub_prop(configuration, ["query", "query"])
 
     @classmethod
     def from_api_repr(cls, resource, client):