[AIRFLOW-5345] Extract SqlSensor get_hook functionality

[gmaliar]

Jira

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Description

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Due to security reasons we have an alternative PostgreSQL hook that fetches credentials dynamically.

We would still like to use the SqlSensor functionality as is as our hook is still backed by sqlalchemy.

By extracting SqlSensor's connection.get_hook into a method we can override the _get_hook method without affecting the functionality of the hook.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

No functionality changed, rather than just extracted to an auxiliary function.

Commits

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Documentation

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Code Quality

• Passes flake8