Introduce a GraphNode class in aai-schema from which all objects inherit

Introduce a GraphNode class in aai-schema from which all objects inherit

- introduce GraphNode class that has the common AAI attributes
  - resource-version
  - data-owner
  - data-source
  - data-source-version
  - relationship-list
- make all AAI classes extend GraphNode
- this provides compiler guarantees for the presence
  of these attributes which is particularly useful for
  the relationships
- bump snapshot versions to 1.13.0-SNAPSHOT

Issue-ID: AAI-4186
Change-Id: Ib89954a2a93dbd4f4865e938236989a5bf99564c
Signed-off-by: Fiete Ostkamp <Fiete.Ostkamp@telekom.de>