Added setup model

Necessary to model the API response.

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

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe TRMNL::API::Models::Setup do
+  describe ".for" do
+    it "answers record for attributes" do
+      attributes = {
+        api_key: "abc",
+        friendly_id: "10CBAF",
+        image_url: "https://usetrmnl.com/images/logo.bmp",
+        message: "Welcome!"
+      }
+
+      expect(described_class.for(attributes)).to eq(described_class[**attributes])
+    end
+  end
+end