Provide a generic mechanism for changing the config loading strategy

[joelgallant]

The @app-config/config module has a loading strategy baked-in. It would be awesome to describe that strategy "declaratively". This way, we can enable choosing different ConfgSources via the meta file.

[joelgallant]

Pseudo-code

FallbackSource:
  - EnvironmentSource:
    - APP_CONFIG
  - CombineSources
    - FlexibleFileSource: .app-config.secrets.{env}.{ext}:
    - FlexibleFileSource: .app-config.secrets.{ext}
    - FlexibleFileSource: .app-config.{env}.{ext}:
    - FlexibleFileSource: .app-config.{ext}

There are some obvious questions here like default values, "merge/combine" strategies with secret files, and parsing extension settings.