Remove need to embed Postgresql credentials in config and provide hooks to integrate with secure keyvault

[rprakashg-redhat]

Currently when you use with Postgresql store for ownership vouchers credentials to connect to pg must be embedded with the db url. In my demo setup my FDO infrastructure was deployed in AWS and using RDS with Postgresql engine. RDS creates an instance of AWS secrets manager and credentials are stored there. It would have been nice to just be able to inject that ARN of Secrets Manager into config and grant permission in AWS IAM to allow the instance running FDO server to read secrets from secrets manager. This also makes rotating credentials easy. Currently one would need to update the config and restart server

Would be great to have hooks into other secure keyvault providers out there in market that customers might be using