Implement events for on-demand revalidation of client pages

Currently the client uses interval-based revalidation for statically-generated pages. Ideally, we want to do on-demand based revalidation. To accomplish this, we need to issue requests to the client when a change is made. Ergo, we need a listener that hits an API endpoint for each instance of the client running in the current environment.

Note: This is dependent on defining the necessary payload for each event.