Airflow-provider-great-expectations

* [FEATURE] Add explicit Airflow version handling - thanks, jeffkpayne!
* [FEATURE] Add example DAG setup conveniences - thanks, jeffkpayne!
* [FEATURE] Defer initialization of DataContext and other resources - thanks, jeffkpayne!
* [DOCS] Update README with virtual env and testing process - thanks, jeffkpayne!
* [MAINTENANCE] Mark additional vars templated in BigQuery operator - thanks, jeffkpayne!
* [MAINTENANCE] Add type hints and update docstrings - thanks, jeffkpayne!
* [MAINTENANCE] Add unit and integration test coverage - thanks, jeffkpayne!

* [BUGFIX] Addressed a bug whereby the fail_task_on_validation_failure in the BigQuery operator was being shadowed by the parent class.
* [ENHANCEMENT] Add support for validation operators when running LegacyCheckpoints with the GreatExpectationsOperator
* [DOCS] Fixed typos in documentation - thanks armandduijn and sunkickr!
* [MAINTENANCE] Make some improvements to the package by updating setup.py dependencies, exposing the example_dags within that, and adding an __init__.py to the example DAG directory - thanks petedejoy and pgzmnk!

* [BUGFIX] Update setup.py with appropriate versions

* [BREAKING] Updated GreatExpectations operator to work with class-based Checkpoints (Great Expectations >= 0.13.9)
* [ENHANCEMENT] Restructured project, provider metadata, examples, added "how to run" steps to README, and added tests

* [ENHANCEMENT] Adding BigQuery operator for easier use with BigQuery and Google Cloud Storage

* [ENHANCEMENT] Adding template fields
* [ENHANCEMENT] Adding fail callback function parameter