Optional callback behaviour for ClusterHelper events.

Implement this behaviour in any module and set it in your config:

config :cluster_helper, event_handler: MyApp.ClusterEvents

All callbacks are optional — implement only the ones you need.

Example

```elixir
defmodule MyApp.ClusterEvents do
  @behaviour ClusterHelper.EventHandler

  @impl true
  def on_role_added(node, role) do
    Logger.info("Node #{node} gained role #{inspect(role)}")
  end

  @impl true
  def on_role_removed(node, role) do
    Logger.info("Node #{node} lost role #{inspect(role)}")
  end

  @impl true
  def on_node_added(node) do
    Logger.info("New node joined the cluster: #{node}")
  end

  @impl true
  def on_node_removed(node) do
    Logger.info("Node left the cluster: #{node}")
  end
end
```