Added current screen response contract

Necessary to validate the current screen API response.

lib/trmnl/api/contracts/current_screen.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require "dry/schema"
+
+module TRMNL
+  module API
+    module Contracts
+      # Validates API response.
+      CurrentScreen = Dry::Schema.JSON do
+        required(:refresh_rate).filled :integer
+        required(:image_url).filled :string
+        required(:filename).filled :string
+      end
+    end
+  end
+end