Added display response contract

Necessary to validate the display API response.

lib/trmnl/api/contracts/display.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require "dry/schema"
+
+module TRMNL
+  module API
+    module Contracts
+      # Validates API response.
+      Display = Dry::Schema.JSON do
+        required(:filename).filled :string
+        required(:firmware_url).filled :string
+        required(:image_url).filled :string
+        required(:refresh_rate).filled :integer
+        required(:reset_firmware).filled :bool
+        required(:special_function).filled :string
+        required(:update_firmware).filled :bool
+      end
+    end
+  end
+end