Add support for models.get and models.list endpoints

[mattt]

See https://replicate.com/docs/reference/http#models.get
See https://replicate.com/docs/reference/http#models.list

Currently, the Model class has only username and name, and the ModelCollection.get method constructs a new instance with the provided username and name arguments.

This PR makes the following changes to bring it more in line with the replicate-javascript and the other official clients:

• Adds url, description, visibility, and other fields to Model
• Adds owner field and reimplements existing username field to deprecated property that aliases this field
• Updates ModelCollection.get to fetch information about the named model from Replicate's API
• Adds ModelCollection.list to fetch public models from Replicate's API
• Refactors run to avoid making an additional fetch for the model
• Updates predictions.create to support creating a prediction by version ID string

[awerks]

model = replicate.models.get(MODEL)

File "/usr/local/lib/python3.10/dist-packages/replicate/model.py", line 136, in get

return self.prepare_model(resp.json())

File "/usr/local/lib/python3.10/dist-packages/replicate/model.py", line 146, in prepare_model

attrs.get("default_example", {}).pop("version", None)

AttributeError: 'NoneType' object has no attribute 'pop'

[mattt]

Thanks for reporting, @awerks. PR with a fix here: #162

[mattt]

@awerks This should be fixed in https://github.com/replicate/replicate-python/releases/tag/0.15.1