Ability to use secrets as environment variables

[worldtiki]

Description

In order to improve the security posture of druid installations using helm chart, I propose this change where environment variables can be set using kubernetes secrets.
This would be useful, for example, when configuring a Postgres extension where one needs to provide credentials.

Example of usage:

envFromSecrets:
  - name: "DATABASE_USER"
    secretKeyRefKey: "databaseSecrets"
    secretKeyRefName: "username"
  - name: "DATABASE_PASSWORD"
    secretKeyRefKey: "databaseSecrets"
    secretKeyRefName: "password"

I'm opening as a draft to get initial feedback. If you agree with the proposal I'll polish the code and update the docs.

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This PR has:

• been tested on a real K8S cluster to ensure creation of a brand new Druid cluster works.
• been tested for backward compatibility on a real K*S cluster by applying the changes introduced here on an existing Druid cluster. If there are any backward incompatible changes then they have been noted in the PR description.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

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