Expose a simple way to add training handlers

Describe the solution you'd like
Need a simple way to add a handler to the training.

Describe alternatives you've considered
Not sure if this is the best way but I managed to override the finalize method as below where I basically called the super then added on to add my handler

    def finalize(self, context):
        super().finalize(context)

        if context.trainer:
            from ignite.engine import Events
            context.trainer.add_event_handler(event_name=Events.EPOCH_COMPLETED, handler=self._myadditional_handler)

Additional context
It would be nice to have a method we can override that we can simply add the handler.
Or even better and cleaner to have the method called to add to the list before we call the trainer as shown in
https://github.com/Project-MONAI/MONAI/blob/d23221f06afdf5fa7d8e3fde6552b6cceaf4a0bc/tests/test_integration_workflows.py#L162-L190

Start with the training flow around the overridable finalize method and compare it with the handler-registration pattern in tests/test_integration_workflows.py at the referenced lines. Determine where a public extension point belongs, then add coverage showing that a custom training handler can be registered before training runs.