Adding simplest configurator. No usage of creds right now

spec/yookassa_spec.rb

@@ -4,4 +4,18 @@ RSpec.describe Yookassa do
   it "has a version number" do
     expect(Yookassa::VERSION).not_to be nil
   end
+
+  describe ".configure" do
+    before do
+      Yookassa.configure do |config|
+        config.shop_id = 123
+        config.api_key = "test_321"
+      end
+    end
+
+    it "stores settings and provides access to credentials" do
+      expect(Yookassa.config.shop_id).to eq(123)
+      expect(Yookassa.config.api_key).to eq("test_321")
+    end
+  end
 end