Added setup endpoint

Necessary to handle the request and response of the setup API endpoint.

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

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe TRMNL::API::Endpoints::Setup do
+  subject(:endpoint) { described_class.new client: }
+
+  include_context "with application dependencies"
+
+  let(:client) { TRMNL::API::Client.new http: }
+
+  describe "#call" do
+    context "with success" do
+      let :http do
+        HTTP::Fake::Client.new do
+          get "/api/setup" do
+            headers["Content-Type"] = "application/json"
+            status 200
+
+            <<~JSON
+              {
+                "api_key": "abc",
+                "friendly_id": "10CBAF",
+                "image_url": "https://usetrmnl.com/images/logo.bmp",
+                "message": "Welcome!"
+              }
+            JSON
+          end
+        end
+      end
+
+      it "answers record" do
+        result = endpoint.call id: "ABC"
+
+        expect(result).to be_success(
+          TRMNL::API::Models::Setup[
+            api_key: "abc",
+            friendly_id: "10CBAF",
+            image_url: "https://usetrmnl.com/images/logo.bmp",
+            message: "Welcome!"
+          ]
+        )
+      end
+    end
+
+    context "with failure" do
+      let :http do
+        HTTP::Fake::Client.new do
+          get "/api/setup" do
+            headers["Content-Type"] = "application/json"
+            status 404
+
+            <<~JSON
+              {"error": "Danger!"}
+            JSON
+          end
+        end
+      end
+
+      it "answers failure" do
+        result = described_class.new(client:).call id: "ABC"
+        expect(result).to match(Failure(be_a(HTTP::Response)))
+      end
+    end
+  end
+end