feat: add cancel method to pipeline client

@@ -264,7 +264,7 @@ def state(self) -> Optional[gca_pipeline_state_v1beta1.PipelineState]:
 @property
 def _has_run(self) -> bool:
 """Helper property to check if this pipeline job has been run."""
-return bool(self._gca_resource.name)
+return bool(self._gca_resource.create_time)
 
 @property
 def has_failed(self) -> bool:
@@ -310,3 +310,19 @@ def _block_until_complete(self):
 log_wait = min(log_wait * multiplier, max_wait)
 previous_time = current_time
 time.sleep(wait)
+
+def cancel(self) -> None:
+"""Starts asynchronous cancellation on the PipelineJob. The server
+makes a best effort to cancel the job, but success is not guaranteed.
+On successful cancellation, the PipelineJob is not deleted; instead it
+becomes a job with state set to `CANCELLED`.
+
+Raises:
+RuntimeError: If this PipelineJob has not started running.
+"""
+if not self._has_run:
+raise RuntimeError(
+"This PipelineJob has not been launched, use the `run()` method "
+"to start. `cancel()` can only be called on a job that is running."
+)
+self.api_client.cancel_pipeline_job(name=self.resource_name)