[bitnami/airflow] Allow customizing the empty-dir volume parameters

[NeonSludge]

Description of the change

This PR makes it possible to customize the properties of the currently hardcoded empty-dir volume used to hold temporary Airflow data (writeable venv, DAGs fetched from git repositories, temporary files, etc.). In addition to a regular emptyDir volume (current behavior) it also provides a choice of a memory-backed emptyDir and a generic ephemeral volume backed by a PVC.

The PR also adds a couple of currently missing empty-dir mounts for the Airflow local settings file (airflow_local_settings.py).

Benefits

Using emptyDir volumes to store what might be a non-negligible amount of data is sometimes not desirable. This change keeps the old behavior by default but provides more options for ephemeral data storage.

Possible drawbacks

Applicable issues

Additional information

Checklist

• [X] Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• [X] Variables are documented in the values.yaml and added to the README.md using readme-generator-for-helm
• [X] Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• [X] All commits signed off and in agreement of Developer Certificate of Origin (DCO)

[NeonSludge]

Force-pushed a version with all the suggested changes.

[jotamartos]

Changes LGTM. Let me update our internal systems and merge the changes.

[github-actions[bot]]

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