Support Flyte as a Model Registry for UnionML Apps

[cosmicBboy]

Flyte provides the building blocks for a fully-fledged model registry, but currently it is more of an arbitrary artifact store. We have:

• an interface to interact with Flyte task/workflow artifacts via FlyteRemote
• all workflow/task executions that produce model artifacts are serialized and effectively versioned in blob store
• the inputs (including config) to those executions are also fully tracked by Flyte

The purpose of this issue is to come up with a thin abstraction layer in UnionML such that e.g.:

• "model.remote_train" executions populate an easily accessible model registry
• unionml exposes an easy way to see what models are associated with a unionml app ("see" needs to tied to TBD requirements)
• as a user, I can compare model artifacts easily and deploy the one that I want to use in prod