feat(weave): Make ref-getting more pleasant

[andrewtruong]

This PR:

1. Adds a mechanism, @objectify.register, 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

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