Added firmware response contract

Necessary to validate the firmware API response.

lib/trmnl/api/contracts/firmware.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require "dry/schema"
+
+module TRMNL
+  module API
+    module Contracts
+      # Validates API response.
+      Firmware = Dry::Schema.JSON do
+        required(:url).filled :string
+        required(:version).filled :string
+      end
+    end
+  end
+end