feat: Create Session.set_message_handler(name, handler)

[schloerke]

Sets a method that can be called by the client via Shiny.shinyapp.makeRequest(). Shiny.shinyapp.makeRequest() makes a request to the server and waits for a response. By using makeRequest() (JS) and set_message_handler() (python), you can have a much richer communication interaction than just using Input values and re-rendering outputs.

For example, @render.data_frame can have many cells edited. While it is possible to set many input values, if makeRequest() did not exist, the data frame would be updated on the first cell update. This would cause the data frame
to be re-rendered, cancelling any pending cell updates. makeRequest() allows for individual cell updates to be sent to the server, processed, and handled by the existing data frame output.

---

Removes Session._output_binding_request_handler() and all methods used to support it.

Now, there is no coincidences and users opt-in directly to the method:

Ex:

        ...
        request_key = session.set_message_handler(
            f"data_frame_patches_{self.output_id}",
            handler_fn,
        )

The request_key is then set to the client so that it know where it can make requests. The request_key is namespaced by the session, so authors do not need to handle it themselves.