Added setup response contract

Necessary to validate API response.

lib/trmnl/api/contracts/setup.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require "dry/schema"
+
+module TRMNL
+  module API
+    module Contracts
+      # Validates API response.
+      Setup = Dry::Schema.JSON do
+        required(:api_key).filled :string
+        required(:friendly_id).filled :string
+        required(:image_url).filled :string
+        required(:message).filled :string
+      end
+    end
+  end
+end