Proposal: Add new optional fields to identify the application that wrote the model #2

The spec only specifies the implementation and implementation_version keys. In an ideal world, every application writing models with the same implementation (for example https://github.com/Stability-AI/generative-models) would output the exact same model. But unfortunately that is not how the real world works.

Two new (optional) keys would be nice: application and application_version

A few things would need to be specified first:

1. What format should the application value be saved in. Is it just a free form text?
2. What format should the application_version be saved in. Ideally something like semantic versioning would be best to make comparisons between versions easy. But maybe that is too strict and only a commit hash is possible.
3. Should these keys be marked as CAN or as SHOULD. Or maybe a combination like application is SHOULD and application_version is CAN.