[bitnami/redis] Support retrieving Redis ACL user passwords from Kubernetes Secrets

[emahdij]

Description of the change

• Introduced auth.acl.userSecret for specifying the secret containing user credentials.
• Added helper function common.secrets.get to fetch user credentials from a Secret.
• Updated Redis ACL user configuration to prioritize secrets over plain-text passwords.
• Ensured backward compatibility by falling back to .password when no secret is found.

Benefits

• Allows passwords to be managed in Kubernetes secrets instead of being hardcoded in values.yaml.

Possible drawbacks

• None. The change is backward compatible, and previous users can continue using the chart without any issues.

Checklist

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