Improve PersistentResourceDefinition to make it easier to register attribute write handlers

Currently if you want to take register custom write handler you need to override whole registerAttributes methods and do it yourself all the way.

We could add PersistentResourceDefinition.getAttributeHandlers() method that returns
a Map<String, OperationStepHandler>.
And then registerAttributes uses the map instead of hardcoding ReloadRequiredWriteAttributeHandler. Default impl just fills the map values with
ReloadRequiredWriteAttributeHandler.