Use existing segmentation as segmentNode

Is your feature request related to a problem? Please describe.
If you have a volume node and a segmentation node already available in slicer and you go to the MONAILabel module, the existing volume node gets recognized and used as master volume, but a new segmentation node is always created.
Due to this it is not possible to start annotating a segment, close slicer and continue later on with the same segmentation or to get a quality check by a second annotator who can then submit the label.

A possible workaround is now to drag the existing segment in the new segmentation node and remove the new empty segment, but it would be nice to be able to just save everything to an .mrb and continue later.

Describe the solution you'd like
This issue can be fixed by a small update in MONAILabelWidget.createSegmentNode() that checks if a segmentation node with the correct name is already available.

def createSegmentNode(self):
      if self._volumeNode is None:
          return
      if self._segmentNode is None:
          name = "segmentation_" + self._volumeNode.GetName()
          if slicer.util.getFirstNodeByName(name):
             self._segmentNode = slicer.util.getFirstNodeByName(name)
          else:
             self._segmentNode = slicer.mrmlScene.AddNewNodeByClass("vtkMRMLSegmentationNode")
          self._segmentNode.SetReferenceImageGeometryParameterFromVolumeNode(self._volumeNode)
          self._segmentNode.SetName(name)

Start at MONAILabelWidget.createSegmentNode(), the entry point named in the issue. Check the existing segmentation node lookup and verify that an available node with the expected name is reused while a missing node is still created; confirm that annotation can be saved and reopened with the same segmentation node.