New mechanics for handling path types

[sfriedowitz]

What's changing

This PR overhauls the path specification mechanics in the library. Instead of using config data classes, I implement a validation type called AssetPath that validates a string for having a prefix in the following:

• hf:// for huggingface repos
• file:// for direct file paths
• wandb:// for fully qualified W&B paths

This approach is analogous to how many other libraries specify paths, and I believe is a more direct way of doing so. It also lets me change the config field from load_from to path, which makes much more semantic sense in my head.

Follow-ups

You'll also notice that I changed the result types from the jobs a bit, to actually include the W&B artifacts directly. In a follow-up, I plan to centralize where the artifact logging is occurring within the LMBuddy class, which should simplify much of the conditional logic that is currently found within the job entrypoints.

[sfriedowitz]

How much of this will we be able to port to the platform if we need to?

I think it will help with the platform a lot! For instance, being able to specify hf:// vs file:// vs s3:// will likely be the difference between raw HF models and internal models that we have trained.

It's good groundwork for some of those goals IMO.