Add heartbeat messages

[Archmonger]

Current Situation

ReactPy currently uses websockets as the primary messaging method for rendering.

This can be problematic because cloud providers (such as Azure or GCP) do not keep web workers running persistently. They are run on demand and will "randomly" restart after a certain idle timeout period, which leads to a bad experience.

Parent issue: reactive-python/reactpy#1259

Proposed Actions

Add heartbeat messages to ReactPy, which will prevent cloud workers from timing out

Ref: https://stackoverflow.com/questions/70215641/how-to-handle-azure-app-service-websockets-timeout