Helm chart: database credentials cannot be loaded from existing secret

### Bug description The structured database credentials introduced in https://github.com/apache/superset/pull/41777 only allow for static values, and have no mechanism to load the password value from a secret. This breaks any workflow that only places database credentials as a secret only, such as CNPG or AWS RDS cluster integration. This also was a problem with the previous credentials under supersetNode, but I have only encountered this now as I switched to an external database with the Superset V6 upgrade. I have partially worked around this by specifying an additional `envFromSecrets:` entry, which worked for the application container but is not passed in to the init `wait-for-postgres` job and that still fails in a loop with the stub password entry in the values. ### Screenshots/recordings _No response_ ### Superset version 6.1.0 ### Python version Not applicable ### Node version Not applicable ### Browser Not applicable ### Additional context This is using the latest released version of the chart, 0.22.4. ### Checklist - [x] I have searched Superset docs and Slack and didn't find a solution to my problem. - [x] I have searched the GitHub issue tracker and didn't find a similar bug report. - [x] I have checked Superset's logs for errors and if I found a relevant Python stacktrace, I included it here as text in the "additional context" section.