Make Python client work for both Cog and Replicate

[bfirsh]

The Replicate HTTP API is the same as the Cog HTTP API. (Or, is intended to be.) It'd be neat if under the hood, there was a single Python client that worked with both Replicate and Cog models.

Some design work is needed here: is it a Cog or a Replicate client? Is Cog some subset of Replicate? Maybe a "version" in Replicate is actually a Cog "model" under the hood? (But then the nouns clash...)