Add credential store source

[lundybernard]

Add a new source that can search a credential store, or config database.

Motivating Example:

• An application may need to access multiple systems using similar credentials (authentication tokens), or one system using multiple users.
• Users probably want to store credentials in a secure, encrypted, service built for the purpose, and pull creds from there, we need an example of this pattern.
• we may want to store multiple configurations for a module in a database or flat-file, and use multiple variations of the same config at runtime.

Example:

the file source Already allows multiple configs at the top-level by setting its config_env parameter. We want a similar mechanism to select a sub-config at runtime.

Related to: lundybernard/project_template/issues/4