feat(chart): support external postgres

[cmdoret]

Context

The chart hardcodes the password secret in values and the postgres hostname in helpers.
This makes it impossible to use external postgres providers that read credentials from existing secrets or use different service names.

Suggestion

• expose values to read password from an existing secret
  • see for example the keycloak helm forge chart existingSecret and existingSecretPasswordKey values
• read postgres host directly from values instead of hardcoding it.