allow overriding of model_name in a state schema (useful for i18n lookups)

[onyxrev]

This PR introduces the ability to provide a custom model_name in a state schema. This is useful when using i18n.

Example:

schema do
  model_name "User"

  attribute :name, :string
  # ...
end

# en.yml

en:
  activemodel:
    attributes:
      user:
        name: "Username"

In the above example, the human_attribute_name for the schema attribute :name would look up the path activemodel.attributes.user.name and would resolve to Username.