What issues does your PR fix?

• resolves specify externalDatabase username from Secret (improves PostgresOperator support) #511

What does your PR do?

• Added values to set externalDatabase/externalRedis passwords directly with plain-text values:
  • externalDatabase.password
  • externalRedis.password
• Added values to set externalDatabase username from a secret:
  • externalDatabase.userSecret
  • externalDatabase.userSecretKey (default: postgresql-user)
• Added warning to NOTES.txt if user sets externalDatabase/externalRedis using the plain-text 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