feat(weave): Make ref-getting more pleasant

[andrewtruong]

Resolves:

• https://wandb.atlassian.net/browse/WB-22632
• https://wandb.atlassian.net/browse/WB-22688
• https://wandb.atlassian.net/browse/WB-22699

This PR:

1. Adds a mechanism, @register_object, to register objects that can be converted from WeaveObject into their base python classes. This can be used both internally and by end-users.
2. Adds a new classmethod from_uri to registered objects
3. Improves ref(...).get() to return the actual python class instead of WeaveObject for registered objects
4. Adds optional refs on all weave.Object which are initially unset, but can be set when the object is re-created from a ref

Now you can do:

ds = Dataset(rows=[...])
ref = weave.publish(ds)

ds2 = ref.get()  # returns a `Dataset` instead of `WeaveObject`
ds3 = Dataset.from_uri(ref.uri())  # returns a `Dataset`

[circle-job-mirror]

Preview this PR with FeatureBee: https://beta.wandb.ai/?betaVersion=896d90b9964e9af637a4553d2fce551b5e90edb3

[andrewtruong]

Not sure if we prefer this or to set obj.__dict__["ref"]

[jamie-rasmussen]

Could we just import the ref method?