Email client, application startup logic and tests

configuration/base.yaml

@@ -6,3 +6,8 @@ database:
   username: "postgres"
   password: "password"
   database_name: "newsletter"
+email_client:
+  base_url: "http://127.0.0.1"
+  sender_email: "sender@example.com"
+  authorization_token: "my-secret-token"
+  timeout_milliseconds: 10000