Implement async versions of the remaining GCS sensors

[kaxil]

Follow up of #35 to Implement async versions of the remaining GCS sensors:

• GCSObjectUpdateSensor -> @bharanidharan14
• GCSObjectsWithPrefixExistenceSensor -> @rajaths010494
• GCSUploadSessionCompleteSensor -> @rajaths010494

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Acceptance Criteria:

• Unit Tests coverage in the PR (80% Code Coverage -- We will need to add CodeCov separately to measure code cov) with all of them passing
• Example DAG using the async Operator that can be used to run Integration tests that are parametrized via Environment variables. Example - https://github.com/apache/airflow/blob/8a03a505e1df0f9de276038c5509135ac569a667/airflow/providers/google/cloud/example_dags/example_bigquery_to_gcs.py#L33-L35
• Add proper docstrings for each of the methods and functions including Example DAG on how it should be used (populate
• Exception Handling in case of errors
• Improve the OSS Docs to make sure it covers the following:
  • Has an example DAG for the sync version
  • How to add a connection via Environment Variable & explain each of the fields. Example - https://airflow.apache.org/docs/apache-airflow-providers-postgres/stable/connections/postgres.html
  • How to use Guide for the Operator - example: https://airflow.apache.org/docs/apache-airflow-providers-postgres/stable/operators/postgres_operator_howto_guide.html

[bharanidharan14]

For GCSObjectUpdateSensor I don't think we need async version operator because in the sync version it just get the blob updated time and check if it is updated, the call is not in poke manner or get_blob won't be blocking call it just get the metadata

[bharanidharan14]

started working GCSObjectUpdateSensor - @bharanidharan14

[kaxil]

Any sensor by design does some polling until a certain criteria is met:

check the source code, it polls until the time is updated on the object @bharanidharan14
https://airflow.apache.org/docs/apache-airflow-providers-google/stable/_modules/airflow/providers/google/cloud/sensors/gcs.html#GCSObjectUpdateSensor

[rajaths010494]

working on GCSObjectsWithPrefixExistenceSensor - @rajaths010494

[rajaths010494]

Working on GCSUploadSessionCompleteSensor- @rajaths010494

[rajaths010494]

#90