Include application and handler_name as additional event handler and upcaster metadata

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Include the associated Commanded Application and event handler name as additional system provided metadata key/values. This allows event handlers to determine the current application and handler name when started with these options configured at runtime.

Example

defmodule ExampleEventHandler do
  use Commanded.Event.Handler
  
  @impl Commanded.Event.Handler
  def handle(event, metadata) do
    application = Keyword.fetch!(metadata, :application)
    handler_name = Keyword.fetch!(metadata, :handler_name)

    :ok
  end
end

Usage:

{:ok, _pid} = ExampleEventHandler.start_link(application: MyApp, name: "ExampleEventHandler1")
{:ok, _pid} = ExampleEventHandler.start_link(application: MyApp, name: "ExampleEventHandler2")

The above demonstrates using the same event handler module but configured at runtime with a unique name so that two event store subscriptions are created and both handlers receive events.