Inbox message icons, JS client SDK, Datadog

We've added message icon customization for inbox messages, enabling users to set different icons for each notification type. The CourierJS client SDK is now available in JavaScript, simplifying the process of incorporating Courier into projects and eliminating the need for an additional back-end service. We've also introduced new API calls, including identify, subscribe, and unsubscribe, as well as a Datadog integration for product notifications data. Improvements have been made to logs, automations testing, template overrides, and other features, addressing bugs and enhancing user experience.