Fire notifications when UID references are created/destroyed

[ramonski]

Description of the issue/feature this PR addresses

This PR adds 2 notification events for UIDReference fields:

• IUIDReferenceCreatedEvent
• IUIDReferenceDestroyedEvent

Code that would like to e.g. reindex the target object when a reference is created/destroyed, might register event handlers like this:

<configure>
  <subscriber
      for="senaite.core.interfaces.IHaveUIDReferences
           senaite.core.events.uidreference.IUIDReferenceCreatedEvent"
      handler=".handlers.on_reference_created" />

  <subscriber
      for="senaite.core.interfaces.IHaveUIDReferences
           senaite.core.events.uidreference.IUIDReferenceDestroyedEvent"
      handler=".handlers.on_reference_destroyed" />
</configure>

The code implementation looks like this:

def on_reference_created(object, event):
    """Reindex the referenced (target) object if needed
    """
    event.target.reindexObject()


def on_reference_destroyed(object, event):
    """Reindex the referenced (target) object if needed
    """
    event.target.reindexObject()

☝️ NOTE
Currently these events are only fired for the new DX based UID reference field

Current behavior before PR

No events fired when a reference between two objects is created/destroyed

Desired behavior after PR is merged

Events fired when a reference between two objects is created/destroyed

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.