Ability to set OnRetry after policy creation.

[Fluttercom]

We are implementing a configuration-based factory for policies, but found no good way of setting OnRetry after a RetryPolicy was created. Changing _onRetry to public property would be helpful in this case.

Also would be nice to remove internal keyword from Engines, to make it possible to extend policies.

What do you think about it? Or maybe is there any other way?