Separate the `App` fields coming from `manifest.json`, `artifact.json` into isolated objects

[sohkai]

A manifest.json and artifact.json comes from off-chain metadata published with each app's version, and I was thinking it'd be nice to encapsulate that information separately so it's more clear what comes from an on-chain source (and is thereby reliable) and what is from metadata (especially if we decide to version the metadata).

[JovianBrowne]

@nivida This issue is worded as more of a suggestion or proposed idea. Is this something that you wanted done?

[nivida]

Yes :-)