Allow user to specify postgresql.auth.password keys etc.

[ilyannn]

The original motivation for only using the existingSecret field was to push people not to store credentials in the yaml files.

However the scenario where user manually puts these values should still be supported, as

1. This the ultimately the user's choice.
2. The security considerations from the hardcoded password can be mitigated with a network policy prohibiting access to bundled Postgres outside of the app.
3. There are plugins that allow one to store a placeholder and then automatically retrieve the credential from an external system.