Add RulesActions for generating event messages

[dannylamb]

Using the EventGenerator service from #445, make three rules actions. One for each write event on a Fedora Resource (Create, Update, Delete).

Here's an example of what the Create action's annotation will look like.

 * @RulesAction(
 *   id = "islandora_generate_create_event",
 *   label = @Translation("Generates a Create event message"),
 *   category = @Translation("Islandora"),
 *   context = {
 *     "entity" = @ContextDefinition("entity",
 *       label = @Translation("Entity"),
 *       description = @Translation("The newly created entity.")
 *     ),
 *   },
 *   provides = {
 *     "message" = @ContextDefinition("string",
 *       label = @Translation("The Create event message")
 *     )
 *   }
 * )

Be sure to set the provided context value instead of returning so the serialized message is available to downstream rules actions.