feat: Support mlflow as a board to read/write model pins

[mconflitti-pbc]

Vetiver and mlflow speak similar languages so it likely possible to view mlflow as a board to pull model versions into a VetiverModel and then deploy as an API. From the user's perspective, things would be more or less the same, but model itself and its metadata, deps, etc. would be coming from an mlflow registry.

[isabelizimm]

This is cool idea! It would likely be some refactoring to reading/writing vetiver models have a configurable backend of MLFlow or pins. Right now, everything is geared towards pins, but if we have access to a model object and some metadata, we should be able to use MLFlow too.

[mconflitti-pbc]

Totally! I was able to get a working example (still relying on pins) that does the mapping from MLFlow concepts to Vetiver. The missing piece would be allowing Vetiver to authenticate against the mlflow server and pull the artifacts it needs from there instead of requiring pins like you said.