Added firmware model

Necessary to model the API response payload.

spec/lib/trmnl/api/models/firmware_spec.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe TRMNL::API::Models::Firmware do
+  describe ".for" do
+    it "answers record for attributes" do
+      attributes = {url: "https://test.io/FW1.0.0.bin", version: "1.0.0"}
+      expect(described_class.for(attributes)).to eq(described_class[**attributes])
+    end
+  end
+end