[FEATURE] send an event before redirecting anything in top context

[kashif-m]

Problem statement
Payment link / SDK integrations where beforeunload event is registered displays a confirmation to the user before redirection.

Aim is to bypass such popups in payment link / SDK integrations.

Proposed solution
Removing event listeners is straightforward and can be done using removeEventListener. But child contexts cannot access cross origin parent's context for removing such event listeners.

Aim here is for the SDK to handle the removal of such event listeners rather than relying on the merchant's to handle this themselves.

This solution offers a straightforward way for the integrators to initialize an event removal subscription offered by the SDK by passing the reference to the function which was used during addition of the beforeunload event.

For example, let's say the integrator used below code for initializing the beforeunload event -

            const fnUsedDuringAddEventListener = function (event) {
                event.preventDefault();
                event.returnValue = '';
            };

            window.addEventListener('beforeunload', fnUsedDuringAddEventListener);

The integrator will also need to include below code snippet in their integration for removing the beforeunload event which is handled by the SDK itself

        <script src="URL TO  HyperLoader.js"
            onload="initializeBeforeUnloadRemoval()">
            function initializeBeforeUnloadRemoval() {
                window.removeBeforeUnloadEventListener(fnUsedDuringAddEventListener)
            }
        </script>

Implementation
Redirections in SDK happen using a common place using the fn replaceRootHref. Before any redirections, trigger postMessage to the top context requesting the SDK in top context to handle the removal of beforeunload events.

References
beforeunload - https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event
postMessage - https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage
removeEventListener - https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/removeEventListener