fix: properly template extraEnv values

[jessebye]

What issues does your PR fix?

#879

What does your PR do?

Runs extraEnv values through tpl so users can utilize templating in their values.

Checklist

For all Pull Requests

• Commits are signed off
• Commits have semantic messages
• Documentation updated
• Passes ct linting

For releasing ONLY

• Chart.yaml version bumped
• CHANGELOG.md updated

[montanapayne]

nice++

[jessebye]

@thesuperzapper Hey, sorry to bother, but would you be able to give this a look? We're currently maintaining a separate fork just for this reason.

[thesuperzapper]

@jessebye please don't cut a release in this PR, only make template changes.

Also, we might want to include a small blurb about this in our docs (which live in markdown files on this repo):

• https://github.com/airflow-helm/charts/blob/main/charts/airflow/docs/faq/configuration/airflow-configs.md

[jessebye]

@thesuperzapper Ok, did a quick rebase and updated the documentation to mention use of templating in extraEnv. Please let me know if there are any other changes you'd like to see.