Add Courier SDK to support multi-channel Communication

Disclaimer: I am an engineer at Courier, so this certainly has my own biases.

TL;DR: Add Courier SDK to support notifications out of the box via a single API invocation.

https://www.courier.com/ provides a single API to communicate across email, SMS, direct messaging and so on. This enables developers to configure their providers of choice (For ex. SendGrid for Email, Twilio for SMS, Discord for direct messaging etc) via a UI, design how they want their notification to look like, and invoke it via a single API call from their tech stack.

Enabling Courier API via this boilerplate will help developers tremendously. We believe you should never build 3 things in your app yourself - Authentication, Payments and Notifications [Ref: https://www.courier.com/blog/the-three-things-to-never-build-in-your-app/]

No files, tests, entry points, or acceptance criteria are named. Start by reviewing the Node.js/Express repository structure and the linked Courier API documentation, then establish the supported integration scope and completion checks before implementation.

Written by the indexing model from the issue text.