Add created_at field to model struct and test

[Ivor]

The Replicate.Models.list/0 function currently breaks due to the API returning created_at as a field in the model response.

** (KeyError) key :created_at not found
    (replicate 1.2.0) anonymous fn/2 in Replicate.Models.Model.__struct__/1
    (stdlib 5.2.3) maps.erl:416: :maps.fold_1/4
    (elixir 1.16.3) lib/kernel.ex:2434: Kernel.struct!/2
    (elixir 1.16.3) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
    (replicate 1.2.0) lib/models.ex:163: Replicate.Models.list/0
    iex:1: (file)

This adds the field to the model struct as well as to the mock client's model response structure. It also updates the Replicate.Models.Behaviour to require the opts keyword instead of the 7 arguments.