Add persistence events

[carson-katri]

This is related to liveview-native/liveview-client-swiftui#1065, and introduces an alternative to native_binding for simplified persistence.

Here's an example of a counter app that persists the count in UserDefaults.

defmodule TestBedWeb.IndexLive do
  use TestBedWeb, :live_view
  use LiveViewNative.LiveView

  @impl true
  def mount(_params, _session, socket) do
    {:ok, assign(socket, count: 0) |> load_persistent_value(:stored_count, "loaded_count")}
  end

  @impl true
  def render(%{platform_id: :swiftui} = assigns) do
    ~SWIFTUI"""
    <Text><%= @count %></Text>
    <Button phx-click="increment">Increment</Button>
    """
  end

  def handle_event("increment", _params, socket) do
    new_value = socket.assigns.count + 1
    {:noreply, assign(socket, count: new_value) |> push_persistent_value(:stored_count, new_value)}
  end

  def handle_event("loaded_count", count, socket) do
    {:noreply, assign(socket, count: count)}
  end
end