[AS7-370] Notification Support

* add notifications to AS7 resources description
  * notifications are described in :read-resource-description operation
    if the notifications boolean is true
  * all resources emit 3 types of notifications by default:
    - resource-added
    - resource-removed
    - attribute-value-written
* NotificationDefinition let any resources describe the notifications
  they can emit by registering them in the ManagementResourceRegistration
[AS7-370] Notification Support

* add test to check notifications in :read-resource-description
  operation
[AS7-370] Notification Support

* add NotificationService to let AS7 resource register/unregister
  notification handlers and emit notifications
[AS7-370] Notification Support

* emit notifications:
  * when a resource is added using AbstractAddStepHandler
  * when a resource is removed using AbstractRemoveStepHandler
[AS7-370] Notification Support

* emit notifications when an attribute value is written using AbstractWriteAttributeHandler
[AS7-370] Notification Support

clean up tests
[AS7-370] Notification Support

refactor notifications handling to send them in a ResultHandler instead
of adding an OperationStep
[AS7-370] Notification Support

* add test to validate notification orders emitted by a composite
  operation
[AS7-370] Notification Support

* expose the NotificationSupport directly from the OperationContext. the
  instance is held by the OperationContext's modelController