config: support overrides from secret providers

[adamdecaf]

PayGate Version: v0.8.0

Description
The config file for PayGate has a few secret values which could come from an external service like Doppler or Hashicorp Vault. These offer audit logs, cross-environment sharing, and other features which are used in production setups. Also it would keep these secrets out of the configuration file.

Layering Config instances on each other comes with a need to handle those overrides is something we could add into the projects which will replace PayGate's existing logic in the future.