[nifi-registry] add support for using a secret to inject flowProvider.postgres.password

[NotAndD]

As the title anticipates, adds the support for using flowProvider.postgres.passwordSecretName and passwordSecretKey to inject the password instead of being forced to write it in plaintext in the values.

This uses the same method used by the flowProvider.git section. Also, similarly to the git section, this calls the appropriate script to replace the properties in the config file so that the database is not by default h2 if flowProvider.postgres.enabled: true