[helm] Support injecting environment variables and external secrets

[fresh-borzoni]

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• I searched in the issues and found nothing similar.

Description

The Fluss Helm chart currently has no way to inject custom environment variables or reference external Kubernetes Secrets/ConfigMaps in the coordinator and tablet server pods.

This prevents using the External Secrets Operator or any other env-based configuration without modifying the chart templates directly.

Willingness to contribute

• I'm willing to submit a PR!