[bitnami/airflow] Add support for plugins loaded from a git repo

[chgl]

Description of the change

This adds support for Airflow plugins to the chart. Plugins can be loaded by specifying a git repo containing the plugin file.

I've moved the setting for the sync interval of both the plugin and the dag files into a single value gitSyncInterval. The main motivation for this was to avoid the complexity introduced if the intervals for the DAG sync and the plugin sync were different: either we'd have to modify the sleep-logic inside the git-repo-syncer bash script, or we'd have to create one git-repo-syncer sidecar for each repo. I'm open to suggestions!

Benefits

Adds basic support for plugins.

Possible drawbacks

The introduction of the gitSyncInterval setting and the removal of the *.interval one may be a breaking change.

Applicable issues

• fixes [bitnami/airflow] support for Airflow plugins #1829

Additional information

We should probably start refactoring the initContainer/git-repo-syncer into its own template as it is quite redundant at this point.

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [bitnami/chart])
• If the chart contains a values-production.yaml apart from values.yaml, ensure that you implement the changes in both files

⚠️ Keep in mind that if you want to make changes to the kubeapps chart, please implement them in the kubeapps repository. This is only a synchronized mirror.

[rafariossaa]

Thank you very much for your PR.

Could you add the changes made to values.yml to the values-production.yml file ?
This two files should only differ in the values of the parameters.

Also, if there is different parameter values between them, you should add that difference to the README.md section for that.